Update Flask-WTF to 0.9.4

Updated to Flask-WTF 0.9.4; note that this requires us to import `wtforms` and `wtforms.validators` ourselves: see https://flask-wtf.readthedocs.org/en/latest/upgrade.html#version-0-9-0 reading: *"Dropping the imports of wtforms is a big change, it may be lots of pain for you, but the imports are hard to maintain. Instead of importing Fields from Flask-WTF, you need to import them from the original wtforms"* (and this applies to the validators as well).

This PR supersedes https://github.com/gae-init/gae-init/pull/47 (update to Flask-WTF 0.9.3) which was pending an official WTForms 2.x release. It seems that Flask-WTF is preparing already for WTForms 2.0 and 0.9.4 may thus provide a (small) stepping stone towards such a future upgrade. Flask-WTF may become obsolete/pointless (that is: to `gae-init` projects) when all required features are in WTForms 2.0, but this is not yet proven.

This PR also proposes to use `from flask.ext.wtf import Form` as this will make it easier in `gae-init-babel` to inject an i18n aware Form using `from i18n import Form`.