Fix translation of WTForms strings

This resolves the https://github.com/gae-init/gae-init-babel/issues/6 *"Strings that are in WTForms are not translated"* issue; it currently builds on top of https://github.com/gae-init/gae-init-babel/pull/8 *"Updated Flask-WTF to 0.9.3"* and https://github.com/gae-init/gae-init-babel/pull/9 *"Updated Flask-Babel to 0.9"* (which are already merged in this pull request).