Clean compile for ReSharper 9

Got a clean compile for ReSharper 9 based on develop.

* Implemented `IUnitTestElementsSource`, which replaces `IUnitTestMetadataExplorer` and `IUnitTestFileExplorer` and some methods in `IUnitTestProvider`.
* Fixed up usage of `UnitTestElementId`
* Fixed up usage of `IUnitTestElementObserver`
* Fixed up creation of categories
* Added `ZoneMarker`
* Renamed assemblies to `...9.0.dll`
* Updated `.nuspec` for the ReSharper 9 layout.

NOTE: The package will have a new ID. This is annoying, but it fixes an issue where uploading a new version of an existing ReSharper 8 package will cause the existing package to no longer be available for download in ReSharper 8. I'm hoping to get a fix into the gallery to prevent the need for this, but can't promise anything...