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...
Would there be any reason why the testem tests couldn't run before the functional tests? They're so slow it's nice to have them go last.