Add WPF support

* Add WpfAutoMeasurement based on WinFormAutoMeasurement
* Delete WinForms project
* Add both WinForms and WPF implementations to the .NET 4.5 project, so
they are both available in the .NET 4.5 nuget
* Refactor AutoMeasurement to a static singleton with public
getter/setter of implementation
* Separate AutoMeasurement to its own file

Notes:
* .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.

* 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.
Add WPF sample app

Based on WinForms sample app
Group both apps in solution folder: .NET 4.5\Samples
Rename conditional symbol WINFORMS to NET45

Fix compile error in WinForms sample app and reused for WPF sample app
Throw exception in ctor if not a WPF application

To help catch misconfigurations or misunderstandings early.
Assign AutoMeasurement.Instance in sample apps
Add XML doc to AutoMeasurement.Instance property
Merge pull request #22 from anjdreas/feature/wpf

WPF support