Enforce type of the XBlocks Fields when set

This updates the XBlock dependency to edX/XBlock@***
and add the newly introduced enforce_type methods
Add xmodule fields enforce_type tests
Make enforce_type an optional flag, to avoid conflicts
enforce-type: set the edx/XBlock#200 merge hash
Add my name to AUTHORS
Merge pull request #3227 from FiloSottile/enforce_type

Enforce type of the XBlocks Fields when set