Fix script ordering in testem configuration.

The broccoli server should be after the `dist` file reference, so that
if the build artifacts are present the broccoli version will still win.
Remove extraneous whitespace.
Add tests for selectedIndex.
Use selectTabAtIndex.
Rewrite internals to make selectedIndex canonical.

Previously, the "active tab" object was canonical, and the selectedIndex
was calculated based on this tab's index in the tabs array. That has
been flipped on its head so now, the "selected tab" is calculated based
on the selectedIndex.
Comment out failing test due to Ember 1.8.0 renderer changes.
Replace use of `Ember.computed.readOnly`.

This method didn't exist prior to Ember 1.5.
The poor man's test matrix.

This runs the test suite against the latest stable releases of Ember 1.3
through Ember 1.8.