Adds mypy linter

[ianepperson]

Add MyPy as an available linter.

Closes #40

[coveralls]

Coverage remained the same at 78.269% when pulling e931957 on ianepperson:develop into 837ecd3 on klen:develop.

[ianepperson]

Looks like MyPy is not available for Python 2.7. It can check 2.7 code, but needs to be run with Python 3. I'll add a conditional in the requirements-test file and make sure the tests run OK when it's not present.

[klen]

@ianepperson Great! Thank you!