add oauth2-phabricator to thirdparty

[ofbeaton]

Wrote a provider for using the oauth2 server capability of the phabricator code review system. It's not a cloud service per se but one you run yourself.

https://github.com/ofbeaton/oauth2-phabricator

I have 100% code coverage and I've already started using it in production.

[coveralls]

Coverage remained the same at 100.0% when pulling 67d0284 on ofbeaton:oauth2-phabricator into 18924c1 on thephpleague:master.

[shadowhand]

I reviewed the code and found a couple of minor issues that could be addressed. Will accept this anyways, since they are non-blocking.

Thanks!

[ofbeaton]

Thanks for merging, and for taking the time to review the code. I've fixed all outstanding issues and performed another release.