storage: Switch to JSON API v1

Google Cloud Storage v1beta2 is deprecated and will be shut down in a couple of months. Please upgrade to v1:

https://developers.google.com/storage/docs/json_api/release_notes#april302014

The migration guide appears to have no issues which affect our use of the API, beyond changing the API base URL:

https://developers.google.com/storage/docs/json_api/v1/how-tos/migrate

This only occurs twice as of 1f2aaf3:

$ git grep -i v1beta2
gcloud/datastore/connection.py:  API_VERSION = 'v1beta2'
gcloud/storage/connection.py:  API_VERSION = 'v1beta2'

Shouldn't we also put this constant in some globals/settings module somewhere? We could even allow people to override it by setting an environment variable or something similar.

Hmm, it is a global really: the 'API_VERSION' is a class attribute of the Connection class in both cases, and is used only by the 'build_api_url()' method of that class as a default when the caller does not pass an argument for 'api_version'.

I can't imagine that the library will continue to work against arbitrary other versions of the API; maybe I'm just not imaginative enough. :)