Fix Flask-Mail hack to make pass unit test.

This commit fixes the TODO item in the test_web.py file that had
a typo in the config variable MAIL_DEFAULT_SENDER (it was written DEFAULT_MAIL_SENDER).
Merge pull request #477 from PyBossa/issue-flask-mail-unittests

Fix Flask-Mail hack to make pass unit test.
Regexp validator for not allowing / char in app.short_names
New validator for non-allowed chars. Closes #474
Add the back slash as a forbidden char.
Add the validator to the username forms.
PEP-8 compliant
Refactor the validators to their own module. Refs #474
Update unit tests for validators.
Merge branch 'issue-474-validators' of github.com:PyBossa/pybossa into issue-474-validators