Fix #26 - add a "Load More Arrivals" option at the bottom of the ArrivalsListFragment list

When pressed, this loads an additional 30 minutes of arrivals.  This process can be repeated to load arrivals in 30 min. increments.  If no additional arrivals are available within the next 30 minutes, a Toast is shown to the user indicating that there were no additional arrivals in the next X hours and Y minutes (which can also be repeated to try the next X hours and Y+30 minutes).  If the list was empty and no additional arrivals were available after being requested, the "no arrivals" text is changed to reflect the updated time window (instead of showing a Toast).

Here is how it looks when arrivals are shown:
![image](https://f.cloud.github.com/assets/928045/1362425/785f2b80-3819-11e3-83eb-699b665a9d56.png)

...and here's how it looks when no arrivals are shown:
![image](https://f.cloud.github.com/assets/928045/1362427/850e20de-3819-11e3-82cc-40d317a802f5.png)