WPF support

Based on WinForms implementation.

Breaking changes:
* .NET 4.5 nuget can no longer assume WinForms will be used. This
means when adding the nuget, one must manually add a one-liner that
assigns either WinFormAutoMeasurement or WpfAutoMeasurement to the
AutoMeasurement.Instance property. The readme would have to be updated
to reflect this. I'd like some feedback on this approach.

Other changes:
* WpfAutoMeasurement tries to report info about screen
where the main window is currently located, and falls back to primary
screen, whereas WinForms always uses primary screen. I found this reasonable, but I'd like some input.