Refactored original tutorial. The original tutorial defined the interface
within ListFragment. I dont like this. It's confusing. So I created a new
package and moved the interface definition to there. MainActivity still
implements the interface. This is done because of access to getFragmentManager.
If a new impl was defined for the interface, getFragmentManager would have
to be passed in. This, while possible, is less clear. So, again, MainActivity
still implements the OnItemSelectedListener interace