Secret songs

Sometimes I want to send song notes to someone but don't want the world to see it!

This is the new form:

![new song](https://f.cloud.github.com/assets/17035/2026867/1404af64-88a2-11e3-80e5-b4612834d5fa.png)

I'm missing a "secret" icon of some sort to make a `visible` and `hidden` song visually different in my dashboard. But this could be added later and wanted a first opinion on the implementation details.

:heart_eyes: