Use docker

Using docker will result in faster build times, as well as faster build *start* times.

Also updated documentation and Make in order to accommodate local development as opposed
to Travis.

Fix Travis builds

Update ease to 0.1.3

Update requirements.

Fixed NoneType value for days_early_for_beta and added a unit test.

TNL-2139

Update pytz to 2015.2

pytz changed their versioning scheme from using letters to using
numbers. The old scheme is screwing up new versions of pip.

Better i18n for due/available time format

Fixed student can't view their submission after recieving grades.

TNL-2000

There are various different such format strings here. But they all result in the same format. So combine them into only two.

Fix a placeholder's translation

'Preview in Latex' was not working in ORA.

This is shown when no text is entered in a response textbox.

TNL-1755

Update NLTK to 2.0.5

Added test for TNL-1758's fix.

TNL-1827

Added beta tester functionality.

TNL-1736

Dispaly rubric-option label instead of name

Fixed training example answer format for old submissions.

TNL-1758

Update assessments format when serializing.

Allow file upload backends other than S3. All tests are passing - merging.

Fix mount directory bug in Vagrantfile

Update translations.

Adds support for multiple prompts in problems.