Run nspecrunner in Single Threaded Apartment

Execute runner in a single threaded apartment to allow tests to use
COM components.

Other test runners run in a single threaded apartment as well.

See mspec
https://github.com/machine/machine.specifications/blob/master/Source/Runners/Machine.Specifications.ConsoleRunner/Program.cs#L21

See nunit
https://github.com/nunit/nunit-console/blob/master/src/nunit-console/Program.cs#L40
Merge pull request #1 from Yesware/nspecrunner-stathread

Run nspecrunner in Single Threaded Apartment