Performance problems

Hi again,

I've noticed a performance problem and I haven't figured out what is causing it yet. I tried attaching the profile but github doesn't allow .cpuprofiles. You can easily reproduce it by profiling the demo table at http://offirgolan.github.io/ember-light-table/#/. The rendering speed isn't that noticeable since you're only rendering ~20 rows off the bat but even at ~150 rows, rendering speed is at ~2.5 seconds. I've reduced my table down to one row and have noticed in the profile that the one row creates about 3 large spikes that have a huge number of function calls. You can see spike after spike, for each row, if you profile the demo page.

I'm continuing to investigate. We really like `ember-light-table` but this performance issue is a show-stopper at the moment. Cheers,

Zach