Convert the methods from GetDefaultCredential() to
get_default_credential() format.

Add get_access_token() method to OAuth2Credentials class (effectively,
all credentials).

Add sign_blob() and get_service_account_name() methods to
ServiceAccountCredentials class.

Change how the private key (in pkcs8 format) is read inside
ServiceAccountCredentials class.

Add pyasn1 and pyasn1_modules as required libraries for the oauth2client
library.