Chore: move DotNetTestNSpec to its own repository

[BrainCrumbz]

Reasons:

Looking at CI build on AppVeyor, everytime the build triggers it will build, package and publish (on proper feed) both NSpec library as well as DotNetTestNSpec.
After a first look, there does not seem to be an easy way to change this behaviour, unless maybe making CI configuration more cumbersome. On the other hand it seems quite common, in other testing frameworks too, to publish one package per repository.

Also, tags on master branch would only refer to NSpec library releases, not DotNetTestNSpec ones.

Needed activities:

The move should affect DotNetTestNSpec code & test projects. DotNetTestSample and NetFrameworkSample projects mainly show how to configure two client projects using NSpec with different frameworks, so they could stay in NSpec repository.

• Create new DotNetTestNSpec repository and initialize it with proper directory structure, readme, license, build scripts, etc
• Move DotNetTestNSpec code and test projects to new repository
• Clean up remaining references from NSpec repository