Markers with arrows

This is an experiment by @boonrs and I that uses the marker icon as a way of showing the line direction. It takes the current markers, replaces the icon with one that resembles an arrow, and then rotates it to point at the next coordinate in the line.

**`master` branch**
![screen shot 2014-05-06 at 5 40 28 pm](https://cloud.githubusercontent.com/assets/51882/2897101/c0d14810-d578-11e3-8b15-18166b201fd5.png)

**`boonrs:markers` branch**
![screen shot 2014-05-06 at 5 42 26 pm](https://cloud.githubusercontent.com/assets/51882/2897104/c95a51ac-d578-11e3-9891-b863d828757c.png)

Remaining Tasks
- [ ] Cleanup the WIP commits.
- [ ] Figure out why [this hack] (https://github.com/boonrs/transitmix/blob/markers/app/js/views/markerview.js#L33-L38) is necessary.
- [ ] Redesign the look of the icons.