Implement natural language API

[dhermes]

• https://cloud.google.com/natural-language/docs/
• https://developers.google.com/apis-explorer/#p/language/v1beta1/
• /v1beta1/documents:analyzeEntities
• /v1beta1/documents:analyzeSentiment
• /v1beta1/documents:annotateText
• Protobufs not released as of https://github.com/googleapis/googleapis/tree/6d6a1529e2c76db85b80c73760ed1791908f3cf2

[dhermes]

When constructing a Document for analysis, the content can either be raw bytes from the request or a reference to an object in a Cloud Storage bucket (gcsContentUri).

@tseaver What approach have we taken in similar situations where one service "reaches into" another? I could construct a method which accepts a reference to a gcloud.storage.blob.Blob or just accept bucket and object names as strings and construct the gs://{bucket}/{obj} string for the user. WDYT?

[tseaver]

The BigQuery LoadTableFromStorageJob is probably the closest equivalent. The other one which is somewhat reltated is the Logging sink to Storage bucket, but there we just pass the user-supplied URI through to the back-end.

[dhermes]

See #2062

[dhermes]

Closed via #2211