Fix issue 70: enable incremental index updates

This modifies Gem::Indexer.update_index as called by Geminabox to fix
the issue documented in https://github.com/rubygems/rubygems/issues/232
Fix ordering-sensitive test result, and note Mac OS X-specific test failure
Add flush_key method to Geminabox::DiskCache
Cache individual gem dependency info rather than API responses

Conflicts:

	lib/geminabox.rb
	lib/geminabox/indexer.rb
Rename disk cache

Conflicts:

	lib/geminabox.rb
Use Marshal as appropriate within cache data

Conflicts:

	lib/geminabox.rb
Just initialize the DiskCache object once
Remove now-spurious sort on each request
Merge branch 'improve_api_caching' into api_caching_on_incremental_update
Selectively flush disk cache based on which gems have changed
Don't rely on / being read only.