Minify now allows static file serving

With slightly altered URLs, Minify can cache files so they're served directly
from the filesystem instead of through PHP. A simple library helps create URLs
and clearing the cache.

See `static/README.md` for details.

static/README.md

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+
+# Static file serving
+
+**Note:** This feature is new and not extensively tested.
+
+Within this folder, Minify creates minified files on demand, serving them without the overhead of PHP at all.
+
+For example, when a visitor requests a URL like `/min/static/1467089473/f=js/my-script.js`, Minify creates the directories `1467089473/f=js`, and saves the minified file `my-script.js` in it. On following requests, the file is served directly.
+
+## Getting started
+
+1. Make sure the `static` directory is writable by your server.
+
+2. In `minify/config.php`, set `$min_enableStatic = true;`
+
+3. Request the test script http://example.org/min/static/0/f=min/quick-test.js
+
+    This will create a new cache directory within `static` and redirect the browser to the new location, e.g. http://example.org/min/static/1467089473/f=min/quick-test.js.
+
+    You should see the minified script and on the server the `static` directory should contain a new subdirectory tree with the static file. Following requests will serve the file directly.
+
+4. Delete the new subdirectory (e.g. `1467089473`) and refresh the browser.
+
+You should be redirected to the new location where the file and cache directory has been recreated.
+
+## Site integration
+
+You don't want to hardcode any URLs. Instead we'll use functions in `lib.php`:
+
+```php
+require_once __DIR__ . '/path/to/static/lib.php';
+
+$static_uri = "/min/static";
+$query = "b=scripts&f=1.js,2.js";
+$type = "js";
+
+$uri = Minify\StaticService\build_uri($static_uri, $query, $type);
+```
+
+If you release a new build (or change any source file), you *must* clear the cache by deleting the entire directory:
+
+```php
+require_once __DIR__ . '/path/to/static/lib.php';
+
+Minify\StaticService\flush_cache();
+```
+
+## URL rules
+
+As URLs result in files being created, they are more strictly formatted.
+
+* Arbitrary parameters (e.g. to bust a cache) are not permitted.
+* URLs must end with `.js` or `.css`.
+
+If your URL does not end with `.js` or `.css`, you'll need to append `&z=.js` or `&z=.css` to the URL. E.g.:
+
+* http://example.org/min/static/1467089473/g=home-scripts&z=.js
+* http://example.org/min/static/1467089473/f=styles.less&z=.css
+
+Note that `Minify\StaticService\build_uri` handles this automatically for you.
+
+URLs aren't canonical, so these URLs are all valid and will produce separate files:
+
+* http://example.org/min/static/1467089473/f=one/two/three.js
+* http://example.org/min/static/1467089473/b=one/two&f=three.js
+* http://example.org/min/static/1467089473/f=three.js&b=one/two&z=.js
+
+## Disable caching
+
+You can easily switch to use the regular `min/` endpoint during development:
+
+```php
+<?php
+
+$query = "b=styles&f=minimal.less";
+$type = "css";
+
+if ($use_static) {
+    require_once __DIR__ . '/path/to/static/lib.php';
+    $static_uri = "/min/static";
+    $uri = Minify\StaticService\build_uri($static_uri, $query, $type);
+} else {
+    $uri = "/min/?$query";
+}
+```