Fix attributes of FileStorage being deleted even if type is FileStorage

- Don't call .lower() and convert() if type is FileStorage
Added testcase for FileStorage type
Fixed python 2 and 3 compatibility problem
Fixed python 2 and 3 compatibility problem, again!
Added tox.ini file to use tox test tool

about tox: http://testrun.org/tox/latest/
merge branch 'devunt/master'
Merge branch 'master' of github.com:twilio/flask-restful