Remove unused source & rename param of getOAuthAccessToken callback

[nguyen-thanhviet-labellevie]

• Remove uid = require('uid2') declaration, since unused anymore.
• Rename param name getOAuthAccessToken callback: params => results for more understandable source code. Refs: https://github.com/ciaranj/node-oauth/blob/master/lib/oauth2.js#L209

[coveralls]

Coverage remained the same at 100.0% when pulling 7082786 on viet-gladd:master into 1ac8cbb on jaredhanson:master.