Remove 'oauth_verifier' from body of requestAccessToken

Remove 'oauth_verifier' from body of requestAccessToken as it’s already being sent in the headers.

See section 3.5.  Parameter Transmission of spec here:
http://tools.ietf.org/html/rfc5849#section-3.5

```   
When making an OAuth-authenticated request, protocol parameters as
   well as any other parameter using the "oauth_" prefix SHALL be
   included in the request using one and only one of the following
   locations, listed in order of decreasing preference:

   1.  The HTTP "Authorization" header field as described in
       Section 3.5.1.

   2.  The HTTP request entity-body as described in Section 3.5.2.
```