Added html5 input field type "month"

[rambowtech]

There was already a widget defined, but no field available. I added it, so the html5 input type "month" may be utilized.

First time I contribute on Github. I hope everything is correct. Feedback would be appreciated.

[azmeuk]

Hello. Thank you for your contribution.
I suppose a new field would require some tests (to be sure it works, and that future contributions wont break it), and some documentation (maybe a reference in docs/fields.rst would be enough).

You can have a look on DateField tests in tests/test_fields.py for instance.

[davidism]

Continued in #593