Added reference to extension library

Added reference to the extension library PHPoAuthUserData to extract user data.
Closes #191
Fixed test to match 34d308c5070384bdb483c59da06ae11510caa397
Merge pull request #197 from robbanl/master

Add support for Harvest
Merge pull request #198 from cmfcmf/patch-1

Fixed test to match latest changes
Merge pull request #194 from lmammino/patch-2

Added reference to extension library
Added new scopes

refs http://developer.github.com/v3/oauth/#scopes
refs https://github.com/blog/1778-webhooks-level-up
Automatic generation of the state parameter in the authorization url for some services plus optional state check when requesting acces token
Merge pull request #199 from cmfcmf/patch-1

Added new scopes
Merge pull request #190 from lmammino/generate-state

Automatic generation of the state parameter in the authorization url
Tweak readme text under extensions