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 ability for runtime to store arbitrary options/metadata
Merge pull request #223 from edx/christina/deprecated

Add deprecated attribute to Field
Adds support for passing JSON parameters to initialize XBlock
Merge pull request #224 from edx/xblock/args

Adds support for passing JSON parameters to initialize XBlock
Enforce type of the Fields when set

Add a enforce_type method and call it on __set__ to make sure that
the stored value type is consistent.

enforce_type is aliased to from_json for the most basic types
Update tests to test Fields setting in addition to loading
Run enforce_type on the supplied field default