"Add crop/guide" buttons

Basically this:

![screen shot 2014-12-09 at 5 22 05 pm](https://cloud.githubusercontent.com/assets/220240/5369388/ee7b3574-7fc7-11e4-819d-2028981631b9.png)

**Why?**
* Draws less attention to the buttons, and encourages adding a guide when there are none.

**Caveats**
* Copy could be tweaked to be consistent (*"add guide"* vs *"create a new guide!"*)
* Not the best CSS in the world… I will roll this into a standard style eventually.

I'd be nice to highlight the new guide button when there are no guides.

Closes #346