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/Strategy/RemoveForeignElementsTest.php

@@ -80,9 +80,6 @@ alert(<b>bold</b>);
     }
 
     function testRequiredAttributesTestNotPerformedOnEndTag() {
-        $this->config->set('HTML.DefinitionID',
-            'HTMLPurifier_Strategy_RemoveForeignElementsTest'.
-            '->testRequiredAttributesTestNotPerformedOnEndTag');
         $def = $this->config->getHTMLDefinition(true);
         $def->addElement('f', 'Block', 'Optional: #PCDATA', false, array('req*' => 'Text'));
         $this->assertResult('<f req="text">Foo</f> Bar');