Pyk3 string compatibility

There are two fixes:

1. Replacing `u''` with `''` to make python3.3 syntax valid, and maintain support for python2

2. Fields templates, could be the case that a field is extended from Layout and this provide full path to the template name.   For example: 'app/custom_field.html' instead of 'app/%s/custom_field.html' where the `%s` argument will be replaced with the template pack in use Django crispy forms should support both cases