Replace AddParameter with HttpContent

Failed to get access token. Following is the fix:

HttpRequestMessage's 2nd param cannot be relative path
The "POST" request created by AddParameter has content-length = 0 according to fiddler. And Instagram returns 400. After switch to using HttpContent to carry the payload ... now it works.
FYI. the other api (e.g. like.post, like.delete) seems to work fine without having to use HttpContent.