Add advanced setting for users to change the server API URL

[barbeau]

In OBA iPhone and Android, there is an advanced setting were a user can change the server API URL. This has proven very useful in testing the apps with early versions of servers before they are ready for production and before they are added to the Regions API.

We'd like to include a similar API URL setting in this app.

The user input for Android and iPhone is in the form of:
api.tampa.onebusaway.org

...and the /api/xxxx is automatically appended to the URL.