Stacked about-to-start notifications

This PR implements stacked notifications for notifications shown right (10 minutes) before a session is about to start.
In case of single session selected:
![device-2014-10-23-093714](https://cloud.githubusercontent.com/assets/3942812/4750232/2356502e-5a90-11e4-86c9-c1293368fae3.png)

In case of multiple sessions selected:
![device-2014-10-23-093800](https://cloud.githubusercontent.com/assets/3942812/4750233/25e896b2-5a90-11e4-8a40-d104d7051b01.png)
![device-2014-10-23-093811](https://cloud.githubusercontent.com/assets/3942812/4750235/2a15c67e-5a90-11e4-82d1-ee5ed770a3f5.png)

*Note:*
The BigText style is ignored with Android L LPX13D. 
With that build every stacked notification will display the room name for every session in the content text, while with android 4.4.4 and previous will be displayed both the room and the speaker name:
![device-2014-10-23-101410](https://cloud.githubusercontent.com/assets/3942812/4750626/00357c0a-5a95-11e4-87c4-7ab72a5903a9.png)