Update the function edit button to match existing icon styles

Left-to-right:
<img src="https://cloud.githubusercontent.com/assets/413693/5128687/8430e028-7091-11e4-932d-becb9d1feedf.png" width="150">
Right-to-left:
<img src="https://cloud.githubusercontent.com/assets/413693/5128689/85e91f98-7091-11e4-8e33-181482a0ab3f.png" width="150">
Functional:
<img src="https://cloud.githubusercontent.com/assets/413693/5128691/8807fa7e-7091-11e4-8c93-4dbee881c00c.png" width="200">
This line isn't needed since `EDITABLE = false` is set below.  I'll remove before merging.