Issue 227

This fixes #227 with the exception of vcxproj support, which would be a new feature.

The PR
  * Adds the ability to open and run tests from a VS project or solution
  * Recognizes all project types previously supported in 2.x: csproj, vbproj, vjsproj and vcproj.
  * Adds support for F# projects (fsproj)
  * Reuses and refactors the old classes from 2.x as well as their tests and adds additional tests.
  * Also refactors the NUnit project support to more closely match VS projects.
  * Moves interfaces to the public API in the expectation that some runners will need them.

I started work on support for vcxproj files - the new C++ file format - which NUnit 2.6.4 does not support. This turns out to be fairly complicated. In fact, it could require a fairly complete emulation of  msbuild to evaluate all properties correctly. At a minimum, more experimentation is needed to determine how the large number of settings available to the user are reflected in the actual project files. 

I have removed the code and I'll create a separate issue and branch for this feature.
This and the next method can be static. Also, I haven't looked, but they look very similar to the code we have in an XML helper class. If that code is available in this project, these methods might be removed.