Update CLA links to developers.google.com

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Allow user to reuse the mutated http.Client to initiate new contexts.
Cache bucket name locally.
This addresses issue #69.
Merge pull request #70 from gmlewis/cache-bucket-name

Cache bucketName locally.
Initial port from the appengine/datastore package, adapted to work with the v1 API

Major changes:
* A specific Client interface is used instead of appengine.Context
* The Multiple parameter in properties has been removed, this is now autodetected
* appengine specific types were moved into the datastore package (GeoPoint, BlobKey)
* Direct support for GeoPoints was removed as it's not supported by the v1 API, they're now serialized like every other struct