Add google.auth.credentials - the public interfaces for credentials (#8)

diff --git a/tests/test_credentials.py b/tests/test_credentials.py
new file mode 100644
index 0000000..d78c554
--- /dev/null
+++ b/tests/test_credentials.py
@@ -0,0 +1,94 @@
+# Copyright 2016 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import datetime
+
+from google.auth import credentials
+
+
+class CredentialsImpl(credentials.Credentials):
+    def refresh(self, request):
+        self.token = request
+
+
+def test_credentials_constructor():
+    credentials = CredentialsImpl()
+    assert not credentials.token
+    assert not credentials.expiry
+    assert not credentials.expired
+    assert not credentials.valid
+
+
+def test_expired_and_valid():
+    credentials = CredentialsImpl()
+    credentials.token = 'token'
+
+    assert credentials.valid
+    assert not credentials.expired
+
+    credentials.expiry = (
+        datetime.datetime.utcnow() - datetime.timedelta(seconds=60))
+
+    assert not credentials.valid
+    assert credentials.expired
+
+
+def test_before_request():
+    credentials = CredentialsImpl()
+    request = 'token'
+    headers = {}
+
+    # First call should call refresh, setting the token.
+    credentials.before_request(request, 'http://example.com', 'GET', headers)
+    assert credentials.valid
+    assert credentials.token == 'token'
+    assert headers['authorization'] == 'Bearer token'
+
+    request = 'token2'
+    headers = {}
+
+    # Second call shouldn't call refresh.
+    credentials.before_request(request, 'http://example.com', 'GET', headers)
+    assert credentials.valid
+    assert credentials.token == 'token'
+    assert headers['authorization'] == 'Bearer token'
+
+
+class ScopedCredentialsImpl(credentials.Scoped, CredentialsImpl):
+    @property
+    def requires_scopes(self):
+        return super(ScopedCredentialsImpl, self).requires_scopes
+
+    def with_scopes(self, scopes):
+        raise NotImplementedError
+
+
+def test_scoped_credentials_constructor():
+    credentials = ScopedCredentialsImpl()
+    assert credentials._scopes is None
+
+
+def test_scoped_credentials_scopes():
+    credentials = ScopedCredentialsImpl()
+    credentials._scopes = ['one', 'two']
+    assert credentials.scopes == ['one', 'two']
+    assert credentials.has_scopes(['one'])
+    assert credentials.has_scopes(['two'])
+    assert credentials.has_scopes(['one', 'two'])
+    assert not credentials.has_scopes(['three'])
+
+
+def test_scoped_credentials_requires_scopes():
+    credentials = ScopedCredentialsImpl()
+    assert not credentials.requires_scopes