Fixed finding dlls to run tests from for NSpecRunner

Now Excludes dlls in /obj/ folder.
Now Excludes NSpec.dll itself which ends up wherever it is used.
Now matches *Spec.dll [not NSpec.dll] as the doc says ends in Spec.

This fixed it finding the Spec dll files in my project.
The previous one never found them at all and only tried to run agains NSpec.dll for me which was not useful.
It seems this could not worked as doc indicates in past for NSpecRunner.