Initial version of integration tests

Part of #73.

These integration tests come from C# template for Visual Studio Extension Package.

`FSharpVSPowerTools.Tests` project uses mocking provided by VS SDK to test VSX behaviours. They should be integrated to FAKE build script. An unfortunate thing is that it uses MsTest so we have to use both NUnit and MsTest runners in the solution. @vasily-kirichenko If you can update FAKE build to integrate this, it would be great.

`FSharpVSPowerTools.IntegrationTests` project runs the package inside Experimental VS instance. I put it into a separate solution to avoid interupting our workflows. At the moment, most of the tests are still failing. I'll learn and try to make it work in next few iterations.

These test skeletons are described at http://dotneteers.net/blogs/divedeeper/archive/2008/02/07/LearnVSXNowPart11.aspx.