Added Library support for sending DMs

Let me know if you prefer:

* Having only one method that takes `url.Values`
* inlining `postDirectMessagesImpl`

Signed-off-by: aditya <dev@chimeracoder.net>
Update directmessages.go

Signed-off-by: aditya <dev@chimeracoder.net>
Shorten direct message (DM) endpoint method names for consistency