Adds support for "password" grant type and the setting of the token value format.

I've added properties for the token value format and grant type.  The token value format is intended to go along with my AFNetworking pull request (https://github.com/AFNetworking/AFNetworking/pull/335).

Although, I could see possibly moving the tokenValueFormat property to the AFHTTPClient class to avoid subclasses needing to override the setAuthorizationHeaderWithToken method.