Add notify type preference.

WIP: still testing to make sure this works as expected

![image](https://cloud.githubusercontent.com/assets/4513759/4461747/f6cedbd2-48bc-11e4-8a20-39533c57a5e6.png)

This adds a preference to not receive all messages, and only mentions/1-1 messages (like the official app) or maybe just 1-1 messages.

Depends on https://github.com/Neamar/notiflow-server/pull/7
That's an interesting concern.

Sadly, just storing this on the `NotificationManager` will not be enough anymore (it led to bugs when the device was low on memory, as Android GC'd the `applicationContext` -- thus the move to persistent SQLite db in https://github.com/Neamar/notiflow-client/pull/8)

Your concern is right however. Thinking back on this, i believe keeping the priority high *after* sending the notification will probably lead to "notification spam".

So for now, let's keep your approach: one head up notification for high priority content, and then back to normal.

You're however opening an interesting debate, and a feature totally unimplemented in Notiflow: threads. They're an important part of Flowdock, but managing them with a 5-line notification is tricky at best.
I had been thinking about different things, but no magic ideas for now.

tl;dr

* Let's keep change as-is
* Let's keep thinking about an optimal way to manage threads