Adding a tox and other configuration files needed in core subpackage.

The coverage RC file for the core subpackage intentionally
leaves out google.cloud._testing. This is because the core tests
don't use the entire functionality of the _testing module, but
the umbrella package does.

By leaving the module in the google-cloud-core package, every
other package can depend on it and have the test helpers
ready to access.

core/.coveragerc

@@ -0,0 +1,13 @@
+[run]
+branch = True
+
+[report]
+omit =
+    */google/cloud/_testing.py
+fail_under = 100
+show_missing = True
+exclude_lines =
+    # Re-enable the standard pragma
+    pragma: NO COVER
+    # Ignore debug-only repr
+    def __repr__

core/tox.ini

@@ -0,0 +1,29 @@
+[tox]
+envlist =
+    py27,py34,py35,cover
+
+[testing]
+deps =
+    pytest
+covercmd =
+    py.test --quiet \
+      --cov=google.cloud \
+      --cov=unit_tests \
+      --cov-config {toxinidir}/.coveragerc \
+      unit_tests
+
+[testenv]
+commands =
+    py.test --quiet {posargs} unit_tests
+deps =
+    {[testing]deps}
+
+[testenv:cover]
+basepython =
+    python2.7
+commands =
+    {[testing]covercmd}
+deps =
+    {[testenv]deps}
+    coverage
+    pytest-cov