Re-added IScenario interface for later extensions. Added interfaces for scenario description classes.
Added SourceLocation to IAssertDescription. Added SourceLocationHelper to find correct source location via StackTrace.
Renamed TypedResultScenario.cs to TypedScenario.cs (as it contains multiple classes).
Enhanced ISpeckDescription and removed subject-initialize arrangement for specks without a subject.
Added possibility to cancel speck runner.
Added initial version of Speck.TestRunner.VisualStudio based on Jonathan Dickinson machine.vstestadapter.
Code cleanup.