Add support for Google Extensible Service Proxy

[maguro]

Cloud Endpoints' Extensible Service Proxy only takes JWTs, not access tokens. This
change provides the ability to trasform UserCredentials and ServiceAccountCredentals
such that they provide JWTs as bearer tokens.

CCLA is in the works. I thought I'd get the ball rolling with the review.

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[maguro]

This PR is for #157.

[maguro]

I wonder if ServiceAccountCredentals needs to customize the forEsp() method since there's ServiceAccountJwtAccessCredentials.

[chingor13]

Sorry for the delay in getting to this. Before we can review this PR, you'll need to sign one of CLA's mentioned above.

[chingor13]

Closing as the CLA hasn't been signed yet so we can't look into the PR yet. Feel free to reopen when the CLA is signed.