Fixes (Python 3.6, tox, StringField default, latest versions...)#49

The latest version which makes use of `StringField` instead of `TextField` introduce some cahnges for those using `StringField` and child because `StringField` wasn't inheriting from `TextField`. Most of changes are simply handled by inheriting of `Field` instead of `StringField` to avoid the brutal `self.data = ''` but for those which are really `StringField` (ie. expected type is a string), the default value was ignored. So this PR properly handle default value for `StringField`. While testing, I noticed some minor glitches I fixed: - Python 3.6 was not supported - it was tested against WTForms 2.0, latest is 2.1 - `tox.ini` wasn't up to date (still testing against Python 2.6) - there was a naming conflict between some test classes