Implemented Constructor Scenarios

Constructor Scenarios allow for specifying that a constructor or factory
method either produce events on an IAggregateRootEntity or throw an
exception when called.
Added sample constructor specs to SampleSource

- Added NUnit support for Constructor specifications
- Added Concert.Plan static method
- Added successful constructor test
- Added constructor test which throws
Fixed up XML comments
Tweak, tweak, said the bird

- Tweaked the xml docs
- Split up the docs because one uber page wasn't cutting it. Still a
sample needs to return into the docs showing off how simple it is to get
started.