Change actionbar icon

Just a proposal, but I think TextSecure looks better with a colorful actionbar icon. Gmail, Hangouts, Whatsapp, K-9 and almost every other app I looked at, are using a colorfull icon as well.

**light theme before/after:**
![light](https://cloud.githubusercontent.com/assets/5296073/3410427/ca193662-fde6-11e3-8ed8-9770cdcea618.png)

**dark theme before/after:**
![dark](https://cloud.githubusercontent.com/assets/5296073/3410428/cb37cc2a-fde6-11e3-88cb-9940d424eccd.png)

// FREEBIE