Add NavigationService from the MVVM light source code.
Remove senseless comments from NavigationHelper.cs
Remove useless using directives from NavigationHelper
Serif font and light theme by default.
Revert "Serif font and light theme by default."

This reverts commit fd7d135a5a3f3469c37fb90060a9634425cba09c.
Serif font and light theme by default.
Little XAML fix on MainPage.
Fixed the using directives in NavigationHelper.
Switched back from NavigationService to NavigationHelper.