Change the default scheduler on RxTestSchedulerAspect to be an invalid scheduler.

- Closes issue #7.
Code cleanup.
Add ContextSpecificationBase.ConfigureContext (to replace client usage of PreTestSetUp - which is meant for internal use).

- fixes issue #8.
Rename ContextSpecificationBase.ConfigureContext to BeforeEstablishContext.

- fixes issue #8.
Add extension methods for TestScheduler methods which only take ticks as a parameter.

- closes issue #6.
Merge remote-tracking branch 'upstream/master'