Cleanup mongodb authentication

Authenticate to the database before getting a reference to the
collection.
Disabled video caption acceptance tests
Merge pull request #1261 from edx/will/disable-caption-tests

Disabled video caption acceptance tests
Merge pull request #1229 from rocha/cleanup-eventtracking-mongo-authentication

Cleanup mongodb authentication
Disable another video caption test
Merge pull request #1262 from edx/will/disable-caption-test-2

Disable another video caption test
Fix bug that omits commits in release email script

The command built to generate the list of commits committed but not
authored by a particular individual was incorrect, so some commits
were incorrectly omitted in the table as entries belonging to the
committer.
Merge pull request #1264 from edx/gprice/fix-release-email-script

Fix bug that omits commits in release email script
Make XModuleDescriptor and XModule act as a single class

By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.
Convert due_date_display_format tests to use factories

Previously, these tests modified due_date, which failed due to the new
restrictions on writing to Scope.settings from the LMS.
Use xblock render shortcuts