Relax open_local_resource rules.

An XBlock developer couldn't figure out why his static files weren't
serving.  At the very least, dots in the filename should be ok.
Merge pull request #217 from edx/ned/relax-resource-url-rules

Relax open_local_resource rules.
Add Set field

All hashable types are converted to sets.
Override the field constructor so that hashable types are converted to
set.
Add tests for Set field