Add composer.json file for easy install via composer.

Composer: http://getcomposer.org/

Since HTML Purifier is not completely psr-0 compatible (a classmap is
not enough for autoloading), the package-description does not contain
anything autoload-related. The user has to include the autoloader
himself.

This lets us create an entry on packagist which allows installing HTML
Purifier without the need to declare a repository in projects; it also
makes it easy to create libraries which want to use HTML Purifier using
composer.

Signed-off-by: Benjamin Steininger <robo47@robo47.net>

composer.json

@@ -0,0 +1,18 @@
+{
+  "name": "ezyang/htmlpurifier",
+  "description": "Standards compliant HTML filter written in PHP",
+  "type": "library",
+  "keywords":  [ "html"],
+  "homepage": "http://htmlpurifier.org/",
+  "license": "LGPL",
+  "authors": [
+  {
+    "name": "Edward Z. Yang",
+    "email": "admin@htmlpurifier.org",
+    "homepage": "http://ezyang.com"
+  }
+  ],
+  "require": {
+    "php": ">=5.2"
+  }
+}