Fix init experiment & create Tests app

[dconathan]

This commit fixes #160 by initializing the butler.experiments collection as a document containing only the exp_uid before calling myApp.initExp.

The problem before was that if someone called butler.experiment.set(key='foo', value='bar') in myApp.initExp, it would initialize the experiment collection as a document which didn't have the exp_uid in it which would create all kinds of hard-to-debug errors down the road.

In this commit, I've also created a Tests app, which is a minimal app for us to write integrated tests for all kinds of stuff (testing the database API, for example). For example, I wrote a test that demonstrates that you can now set an arbitrary key using butler.experiments.set in myApp.initExp.

I've run all tests locally (including new ones) with no errors.

[stsievert]

LGTM, and 👍 to the test app.

[erinzm]

@lalitkumarj is there any reason this PR hasn't been merged yet?