Migrations!

[thedodd]

TODO:

• [x] get some serious docs in place.
• [x] a few needed updates to Model::sync to remove expect statements.
• [x] matrix of tests against various MongoDB versions. Probably just >= 3.2.
• [x] fix README image.
• [x] test coverage!
• [x] open issue against mongo driver on WriteErrors being nested inside of the Ok portion of Results.

Closes #3. Closes #4.