Implement %AutoFormat.RemoveEmpty, end to start ref, and injector rewind.

Injector rewind: Injectors can now use the method rewind() in order to move
the input index backwards, so that they can reprocess tokens (other injectors
are not affected by a rewind). This functionality was necessary to implement
nested node removals in %AutoFormat.RemoveEmpty.

End to start ref: To facilitate rewinding, HTMLPurifier_Token_End now
maintains a reference called $start to the starting token for their node.

%AutoFormat.RemoveEmpty removes empty nodes. Lots of people have requested
it, so here is a partially effective implementation. Because it is implemented
as an Injector, it's not possible for it to handle newly introduced empty
nodes by later validators, specifically auto-closing and child validation.
The Injector is only meant to be used on HTML-ish languages.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/HTMLPurifier/Injector/RemoveEmptyTest.php

@@ -0,0 +1,58 @@
+<?php
+
+class HTMLPurifier_Injector_RemoveEmptyTest extends HTMLPurifier_InjectorHarness
+{
+    
+    public function setup() {
+        parent::setup();
+        $this->config->set('AutoFormat', 'RemoveEmpty', true);
+    }
+    
+    function testPreserve() {
+        $this->assertResult('<b>asdf</b>');
+    }
+    
+    function testRemove() {
+        $this->assertResult('<b></b>', '');
+    }
+    
+    function testRemoveWithSpace() {
+        $this->assertResult('<b>   </b>', '');
+    }
+    
+    function testRemoveWithAttr() {
+        $this->assertResult('<b class="asdf"></b>', '');
+    }
+    
+    function testRemoveIdAndName() {
+        $this->assertResult('<a id="asdf" name="asdf"></a>', '');
+    }
+    
+    function testPreserveColgroup() {
+        $this->assertResult('<colgroup></colgroup>');
+    }
+    
+    function testPreserveId() {
+        $this->config->set('Attr', 'EnableID', true);
+        $this->assertResult('<a id="asdf"></a>');
+    }
+    
+    function testPreserveName() {
+        $this->config->set('Attr', 'EnableID', true);
+        $this->assertResult('<a name="asdf"></a>');
+    }
+    
+    function testRemoveNested() {
+        $this->assertResult('<b><i></i></b>', '');
+    }
+    
+    function testRemoveNested2() {
+        $this->assertResult('<b><i><u></u></i></b>', '');
+    }
+    
+    function testRemoveNested3() {
+        $this->assertResult('<b> <i> <u> </u> </i> </b>', '');
+    }
+    
+}
+