Adding NSMutableURLRequest+OAuth2 & Tests
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Handle multiple-requests failing OAuth issue

- The credential property is now atomic.
- A refreshingToken property (atomic) is used to prevent double refresh.
- On 401 failures now, we check and make sure we’re not refreshing the 
  token and that the failing token is the current token before we 
  refresh the token. Otherwise we just fail the request.
- Now using convenience methods for setting/getting bearer tokens