Switch event creation method, part of #43

The user can create new events by selecting a region and then pushing either `enter` or `ctrl-n`. 

![screen shot 2014-10-25 at 11 15 57 pm](https://cloud.githubusercontent.com/assets/6354033/4782058/ed2882c0-5cd7-11e4-87c6-8e0053e07989.png)

![screen shot 2014-10-25 at 11 16 03 pm](https://cloud.githubusercontent.com/assets/6354033/4782060/f54a094c-5cd7-11e4-9b6e-4fff928eaa61.png)

As you can probably tell from the screenshots, these changes rely on the ideas + fixes pushed in the row height adjustment PR #49.
Still attach a single listener to `timeline_svg`, which delegates events down to the markers.