Build against the appengine runtime.
Merge pull request #47 from rakyll/build

Build against the appengine runtime
Locally cache oauth tokens.
This change is for both App Engine and Managed VMs so that these apps can scale without running into the app_identity_service quota limit due to calling appengine.AccessToken too frequently.  An added benefit of caching is that calls to Google APIs will be significantly sped up due to removing the round-trip calls to the api_identity_service.
Add Memcache caching.
Implement clock-skew caching safety margin of 5 minutes to match Python and Java runtimes.
Do not modify original expiry time.

This change was recommended by Peter.
Add unit tests for appenginevm_test.go