Integrate google-auth-library

[stephenplusplus]

Fixes#376

Todos

• Test in environments: (Priority order: Simple authentication when running in GAE + GCE (or locally via Cloud SDK) google-cloud-ruby#102)
  • With config.credentials
  • With config.keyFilename path to a JSON file
  • With config.keyFilename path to a p12 file + config.email
  • With config.keyFilename path to a pem file + config.email
  • With GOOGLE_APPLICATION_CREDENTIALS environment variable
  • With gcloud auth generated keyfile (gcloud sdk)
  • GAE Prod
  • GAE Dev via GOOGLE_APPLICATION_CREDENTIALS environment variable to a JSON file in the project
  • GCE
• Get PR merged on google-auth-library
• Get another PR merged on google-auth-library
• google-auth-library needs a new release
• Unit tests
• System tests for the following configurations:
  • With config.credentials
  • With config.keyFilename path to a JSON file
  • With GOOGLE_APPLICATION_CREDENTIALS environment variable
• Docs: http://stephenplusplus.github.io/gcloud-node/#/docs/master

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[googlebot]

CLAs look good, thanks!

[stephenplusplus]

Silly CLA-bot buddy.

[stephenplusplus]

@ryanseys I'm sorry for the enormous PR. It's ready for a look while we're waiting for changes to google-auth-library.