Added ability to pass an argument to the authorize_token_url call so the...

... client can request the mobile version of the fitbit authorization.  Would make my users a load less squinty.  Tox tests pass for the versions of python I have installed:
```python
  pypy: commands succeeded
  py34: commands succeeded
ERROR:   py33: InterpreterNotFound: python3.3
ERROR:   py32: InterpreterNotFound: python3.2
  py27: commands succeeded
  py26: commands succeeded
```