Create NewExpense View.

Create NewExpense View and ViewModels, and wire up to the preliminary
CreateExpense proxy method. Also small refactorings to the FriendViewModel
to make it consistent with the NewExpenseViewModel.
Merge branch 'master' of https://github.com/tmrudick/splitwise-wp8

Conflicts:
	splitwise-wp8/Views/Home.xaml
	splitwise-wp8/splitwise-wp8.csproj