Add tooltips to action buttons

It's not clear to users what the line direction icons mean. Add tooltips to make it clearer.

![screen shot 2014-09-18 at 1 32 26 pm](https://cloud.githubusercontent.com/assets/1521387/4319148/3f96c238-3f28-11e4-89a4-bf852684c941.png)

![screen shot 2014-09-18 at 1 36 40 pm](https://cloud.githubusercontent.com/assets/1521387/4319149/40a196e4-3f28-11e4-9436-2a0248169fda.png)