display the messages in a table in the app

We really enjoy using the messages feature and thought it would be nice to be able to see them in the app as well. There is an [accompanying PR on the server](https://github.com/panicinc/PunchClockServer/pull/8) as well.

This is what it looks like now:

![img_1169](https://cloud.githubusercontent.com/assets/749864/3656273/76eecd92-1184-11e4-81e7-d8aaab23ba02.PNG)


I've pulled out some common code into `PCTableViewController.h` that both the `PCStatusTableViewController` and the new `PCMessagesTableViewController` use, I was considering pulling out some of the date fetching code into it's own class as well since there is some duplication there now but I figured it was still small enough. Let me know if you'd prefer that though!