added Preposition button

You can see the Preposition button in Phrases lesson 8.

N.B.  It is VITALLY important that you delete the previous copy of the
GrammarTrainer app from your iPad simulator before running the new
version of GrammarTrainer.
fixed empty answer bug.
Merge branch 'master' of https://github.com/katiebeals/SWGT

Conflicts:

GrammerTrainer.xcodeproj/project.xcworkspace/xcuserdata/brandall1.xcuser
datad/UserInterfaceState.xcuserstate