Evaluate content before passing to regex.sub

Issue #1708
evaluate content at function start
remove dep on python_2_unicode_compatible

python_2_unicode_compatible is not available in all Django versions
replace unicode call with force_text
Add subclass matching to serializer field mapping.
Traverse the method resolution order when mapping serializer fields.
Add utility function to match classes in dictionary.
Generator implementation of class mapping.
Display validation errors on login form
Error messages are displayed below the input fields
General message error goes above submit button
Input fields get hightlighted if login fails and username is persisted
Change bootstrap class warning to error
Place general error message in well
Fix returning None when allow_none is True in CharField
Check object in tests
Fix comment
Remove light gray bordering from the well with general message error
Merge pull request #1829 from cezar77/login-form-errors

Display validation errors on login form
Use Serializer instead of ModelSerializer