Fix for breaking update made Fitbit in OAuth API

Fitbit released a series of changes to the OAuth steps. As of 2 days ago
this library become non-functional, and this update contains breaking
changes. To update your code see this:
https://github.com/aarondcoleman/Fitbit.NET/wiki/Breaking-Change-on-1-24-2014-as-a-result-of-OAuth-update-in-Fitbit-API