Better handling of oauth_callback parameter to requestToken endpoint

Hi Michael,

I ran into an issue in using this library to call the requestToken endpoint with an oauth_callback URL that itself contains parameters. Since the existing code is just doing a GET against the requestToken endpoint and appending oauth_callback as a URL parameter, when the callback URL itself has parameters, they get mixed up. So one fix would just be to percent-encode this; but since the LinkedIn Authentication docs suggest POST is the preferable method, I updated the code to send the oauth_callback in the POST body instead, and this works for me now.

By the way, ignore the other pull request I sent before, this one replaces it.