Replace deprecated flask-wtf references to 'From' with 'FlaskForm'.

[jcass77]

The Flask-WTF project has renamed the 'Form' class to 'FlaskForm', and the deprecated reference will be removed in version 1.0.

A side effect of this was to reveal an error in the __init__ method signature used in the Stormpath-Flask code:

stormpath-flask/flask_stormpath/forms.py

Lines 37 to 40 in ce155b2

	def __init__(self, formdata=_Auto, obj=None, prefix='', csrf_context=None, secret_key=None, csrf_enabled=None,
	config=None, *args, **kwargs):
	super(RegistrationForm, self).__init__(formdata, obj, prefix, csrf_context, secret_key, csrf_enabled, *args,
	**kwargs)

(the *args argument causes a TypeError: __init__() takes at most 2 arguments (7 given)).

This PR switches to the new form name, and bumps the version dependency on Flask-WTF to 0.12.