Made updates to match golang/oauth2 latest (breaking) changes.

See breaking change here:
https://github.com/golang/oauth2/commit/0ae3d4edc99f765dc1513a9605aa96e82877a20a
Broken build caused by new oauth2 commits fixed.
Check for empty code in calback

Prevent a redirect loop by checking for an empty `code` GET parameter
in the callback request.
Merge pull request #4 from tcolar/token_extra

Made updates to match golang/oauth2 latest (breaking) changes.
Merge branch 'denizeren-master'

Conflicts:
	oauth2.go
Merge branch 'master' of https://github.com/mattbostock/negroni-oauth2 into mattbostock-master

Conflicts:
	oauth2.go
Merge branch 'mattbostock-master'
Merge branch 'master' of https://github.com/minjatJ/negroni-oauth2 into minjatJ-master

Conflicts:
	oauth2.go
Merge branch 'minjatJ-master'

Conflicts:
	oauth2.go
Update contributors in readme
Updating example
updated read
Use a random state code in AuthCodeURL