Adding capability to use ChoiceField with upper case string value

Signed-off-by: Alexander Lehnert <alex.lehn@googlemail.com>
Merge pull request #5 from alexlehn/pullrequests/upper-case-ChoiceField

Adding capability to use ChoiceField with upper case string value
Fixed the python3 types. The builtin basestring abstract type was removed
Merge branch 'develop' into django_support