Add initial client unit test.

Adds basic client unit testing setup using [ember-mocha](https://github.com/switchfly/ember-mocha).

There are many example blueprints for unit tests using ember-cli-mocha [here](https://github.com/switchfly/ember-cli-mocha/tree/master/blueprints). These are written for Ember CLI, but apply nearly the same here.

My hope is that this is just the beginning of many unit tests for the admin client.

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Running `testem` in the root of the project brings up the testem runner:

![screenshot](http://monosnap.com/image/MZrn1VC5In2SNnUvteEtVSulSnMxis.png)

Navigating to http://localhost:7357/core/test/client/index.html then shows the browser tests:

![screenshot](http://monosnap.com/image/mD9qbMtEht6p0xmXqHZU2yb8U4UXxx.png)

Addresses #2990.

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p.s. Sorry it took me so long...