fix token exchange
some pr review fixes
Merge pull request #36 from jtolds/master

fix token exchange
Merge pull request #37 from stacktic/DropboxAuthFix

Add Dropbox to the providers not supporting Auth headers.
Fixing build constraints for GAE and GAE managed runtime.

GAE managed VM runtime contains both appengine and appenginevm
build tags. Currently, appengine.go is being built even if user
wants to use the package on managed VM that errors with no
"appengine" package to import.
Travis build should keep building appenginevm configuration.
Build should use the exact build tag configuration of GAE managed VMs.
Add Linkedin to the providers not supporting Auth headers.
Merge pull request #39 from clsung/master

Add Linkedin to the providers not supporting Auth headers.
Merge golang/oauth2 master