Update lib/oauth.js

Fix the mismatch between the output of querystring.stringify() and this._encodeData(). 
Add in Authorization Header and support extra headers by default.
Merge pull request #116 from yaru22/master

Adding post_headers parameter to getOAuthAccessToken method.
Merge pull request #114 from rolandboon/master

Fix requests containing !'()* in POST data
Merge https://github.com/ciaranj/node-oauth