Added message category to notifications for Android 5 silent mode.

Hi,

I've noticed that textsecure notifications are not treated as messages in Android 5, meaning they won't show up if you configured "silent mode" (see settings -> sounds & notifications -> interruptions). From what I read from the docs, settings the category appropriately should do the trick. That said I currently don't have a build environment setup, but I'd appreciate some feedback on whether this is a good idea or not. If you like it I'll try to test this on my phone first.