Add a new icon

The first thing I noticed when using TextSecure was that the icon doesn't follow [Android iconography guidelines](http://developer.android.com/design/style/iconography.html). They suggest to:

> Use a distinct silhouette. Three-dimensional, front view, with a slight perspective as if viewed from above, so that users perceive some depth.

I took the current icon, applied the [suggested best practices](http://android-developers.blogspot.com/2013/07/making-beautiful-android-app-icons.html), and came up with this. What do you think? This is just a first pass, but I'm happy with the results.

![screenshot_2013-12-12-16-36-46](https://f.cloud.github.com/assets/34893/1738950/d3eb3930-638e-11e3-9cce-103da50462f2.png)

PS: Ignore the failed Travis build, it doesn't mean anything.