PyLTI edX LTI 2.0 support
Update per review
Add `@property`
Adding whitespace
Removing whitespace
Update imports
Change edge.edx.org to example.edu
Add new lines
Adding note to post_message2 related to https://openedx.atlassian.net/browse/PLAT-281
Merge pull request #6 from mitodl/feature/lti_2_edx

READY FOR REVIEW: PyLTI edX LTI 2.0 support