Parse HTML with libxml2's SAX parser

Results in a 2.5x performance improvement to doc set indexing.
Remove Objective-C-HTML-Parser
Move symbol scanning to its own method
Fix whitespace in FSEventStreamCreate()
Index HTML files in parallel

Results in significantly faster doc set indexing.
Use UTTypeConformsTo() for HTML file test
Add comment for each anchor format
Report index begin/end for individual doc sets