Add service_account.Credentials.to_jwt_credentials (#45)

diff --git a/google/oauth2/service_account.py b/google/oauth2/service_account.py
index dfbe352..df6f1b2 100644
--- a/google/oauth2/service_account.py
+++ b/google/oauth2/service_account.py
@@ -204,6 +204,33 @@
             filename, require=['client_email', 'token_uri'])
         return cls._from_signer_and_info(signer, info, **kwargs)
 
+    def to_jwt_credentials(self):
+        """Creates a :cls:`google.auth.jwt.Credentials` instance from this
+        instance.
+
+        The new instance will use the same private key as this instance and
+        will use this instance's service account email as the issuer and
+        subject.
+
+        This is the same as calling
+        :meth:`jwt.Credentials.from_service_account_file` with the same
+        file used to create these credentials::
+
+            svc_creds = service_account.Credentials.from_service_account_file(
+                'service_account.json')
+            jwt_from_svc = svc_credentials.to_jwt_credentials()
+            # is the same as:
+            jwt_creds = jwt.Credentials.from_service_account_file(
+                'service_account.json')
+
+        Returns:
+            google.auth.jwt.Credentials: A new Credentials instance.
+        """
+        return jwt.Credentials(
+            self._signer,
+            issuer=self._service_account_email,
+            subject=self._service_account_email)
+
     @property
     def requires_scopes(self):
         """Checks if the credentials requires scopes.