Log Message:
Update py-oauth2client to 2.0.0.

## v2.0.0

* Add django_util (#332)
* Avoid OAuth2Credentials `id_token` going out of sync after a token
  refresh (#337)
* Move to a `contrib` sub-package code not considered a core part of
  the library (#346, #353, #370, #375, #376, #382)
* Add `token_expiry` to `devshell` credentials (#372)
* Move `Storage` locking into a base class (#379)
* Added dictionary storage (#380)
* Added `to_json` and `from_json` methods to all `Credentials`
  classes (#385)
* Fall back to read-only credentials on EACCES errors (#389)
* Coalesced the two `ServiceAccountCredentials`
  classes (#395, #396, #397, #398, #400)

### Special Note About `ServiceAccountCredentials`:
-------------------------------------------------

For JSON keys, you can create a credential via

```py
from oauth2client.service_account import ServiceAccountCredentials
credentials = ServiceAccountCredentials.from_json_keyfile_name(
    key_file_name, scopes=[...])
```

You can still rely on

```py
from oauth2client.client import GoogleCredentials
credentials = GoogleCredentials.get_application_default()
```

returning these credentials when you set the `GOOGLE_APPLICATION_CREDENTIALS`
environment variable.

For `.p12` keys, construct via

```py
credentials = ServiceAccountCredentials.from_p12_keyfil(
    service_account_email, key_file_name, scopes=[...])
```

though we urge you to use JSON keys (rather than `.p12` keys) if you can.

This is equivalent to the previous method

```py
# PRE-oauth2client 2.0.0 EXAMPLE CODE!
from oauth2client.client import SignedJwtAssertionCredentials

with open(key_file_name, 'rb') as key_file:
    private_key = key_file.read()

credentials = SignedJwtAssertionCredentials(
    service_account_email, private_key, scope=[...])
```