enabling usage with access token only

[juliusse]

Fixes #1346
This pull request enables gloud-node users to only specify an access_token for authentication.
As @stephenplusplus suggested, the solution just not uses the auto-auth client, but creates a simple one that just sets the access_token.

[coveralls]

Coverage remained the same at 100.0% when pulling 0eaa80a on juliusse:enabled-auth-with-access-token into 24810b9 on GoogleCloudPlatform:master.

[stephenplusplus]

Thanks for this :) There are a couple edge cases, though:

1. Calls made with gRPC APIs won't use the token (Datastore, Logging, Pub/Sub for now)
2. There are other libraries we use-- gce-images & gcs-resumable-upload-- that would need to be updated to accept just an access token (and we would have to provide the access token the user give us to those libraries)

@murgatroid99 is there a way to provide gRPC with just an access token?

[stephenplusplus]

I'm going to close the PR (thank you, though!) and let the discussion carry on in #1346. As mentioned above, it's rather tricky to implement.

[sedouard]

Now that gRPC can easily set metadata (and therefore the access_token) is it in the plan to add access_token to the RESTful interfaces?