#43 Change TabBar to SASlideMenu

Along with changing the TabBar, there were plenty of minor changes that needed to happen
to keep everything working properly. *The code is self explanatory.
Also, I re-included the constants file so users can git clone without issues. (dummy API keys in file)
Add Constants file/Fix Edit bug

Constants file wasn't added last commit like I thought it was.
Now it is.
Edit bug appeared when deleting the last item in a shopping list with a
Swipe and delete gesture.
The bug made the CoreData fetch controller try to reload empty
indexes.