Enable error pages (404.html and 500.html)
sites can now have an _includes directory

It works exactly like _layouts
Fix inconsistent behavior in 404/500 handling
turns out, this now requires the _includes work merged
Merge branch '_includes_dir' into error_pages
extend error page search path to include both

_includes and _layouts
Merge branch 'master' into error_pages
fix build_search_path and it's tests

Because of recent changes, a call to build_search_path without
either an 'append' argument or include_start_directory set to True
will return an empty list.

I no longer recall why "include_start_directory" is called that. Let's
think of a better name.
removed a PDB
Merge pull request #46 from rosskarchner/error_pages

Enable error pages (404.html and 500.html)