Fix two bugs with caching of customized raw definitions.

The first bug is that we will repeatedly write out the result
of a customized raw definition to the filesystem, even when a cache
entry already exists.

The second bug is that caching these definitions doesn't actually
work (the cache entry is written but never used.)  A new API
for retrieving raw definitions permits the user to take advantage
of caching.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

tests/HTMLPurifier/DefinitionCache/SerializerTest.php

@@ -172,6 +172,7 @@ class HTMLPurifier_DefinitionCache_SerializerTest extends HTMLPurifier_Definitio
      * Asserts that a file does not exist, ignoring the stat cache
      */
     function assertFileNotExist($file) {
+        clearstatcache();
         $this->assertFalse(file_exists($file), 'Expected ' . $file . ' does not exist');
     }