Update service account authentication

[kunanit]

Updates methods for service account authentication:

• Removes use of depreciated SignedJwtAssertionCredentials from oauth2client (resolves #66).
• Google recommends using .json keyfiles instead of .p12, so support for .json service account keyfiles is implemented. If a .json keyfile is used then the auth_service_acct setting in edx2bigquery_config is not required.
• So that those with .p12 keyfiles don't have to generate a new keyfile and update settings, authentication with .p12 keyfiles is reimplemented, but uses the oauth2client.service_account.from_p12_keyfile method. The HAS_CRYTO variable / "Try installing OpenSSL" warning is removed because oauth2client already throws a custom exception prompting the user to install PyOpenSSL if it is not found.

[ichuang]

Thanks! Been meaning to do this for awhile.