Add support for dashclock lockscreen widget.

This commit includes support for popular dashclock lockscreen widget.
Dashclock widget playstore url:
https://play.google.com/store/apps/details?id=net.nurik.roman.dashclock
When the above dashclock widget is installed, users of K9 will be able
to see unread message count on the lockscreen. It also includes a
setting activity to choose email accounts to show(only email/
description and unread count) in expanded view of dashclock extension

![dashclock_explained](https://f.cloud.github.com/assets/227169/1252460/aa39dc30-2b4a-11e3-9331-ff11f3f664a2.png)