Fix Unicode Decode error for FileBrowserFiled under Mysql and FileObject.__repr__

When using MySQL backend and try to save file with unicode symbols in his name it raise unicode decode error. The reason is because `future.builtins.str` does not correctly encode the string. For that reason we need to use `smart_str` from `dajngo.utils.encoding`.

And also `__repr__` method need to return `str` object in both Python2 and Python3. But when `unicode_literals` are imported in Python 2 it returns `unicode` object. To fix that I use `smart_str`.