Added Library support for sending DMs
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 <34c6fceca75e456f25e7e99531e2425c6c1de443@chimeracoder.net>

amit-lulla/twitterapi,jeffk/anaconda,rendon/anaconda,ifross89/anaconda,intfrr/anaconda,jmptrader/anaconda,Fe4nn0r/anaconda,azr/anaconda,josephyzhou/anaconda,cgag/anaconda,logashoff/anaconda,140proof/anaconda,rfsbraz/anaconda,aatishnn/anaconda