Allow implicit dataset / bucket / other behavior from environment

[dhermes]

Not necessarily needed for M1, just points in the direction of running in environments like GAE and GCE.

I took a stab at it using environment variables in dhermes@0860e59
In #444 this is done for datasets.

@craigcitro I tried to parse how getting auth values from the environment works in GetCredentials and BaseApiClient.__init__ but it was unclear (for example) how _API_KEY is set.

[craigcitro]

You can actually ignore _API_KEY -- for Cloud, it's basically unneeded. (It's still needed in a few internal use cases, so it needs to stay available.)

GetCredentials otherwise just steps through each of service account, GAE, GCE, looking to get credentials, and finally bottoms out in the usual oauth2 flow.

[silvolu]

I've been talking with Craig to move parts of GetCredentials to oauth2client; after that we can rework the auth bits of this lib. Tracked in #335.

[dhermes]

Just renamed this issue from Allow implicit auth from environment

The auth parts will be handled in #335

The remaining will be dataset / etc. specific so certain things "just work".