Added possibility for better Unit Test some of the Visual Studio Dte using things like  Intellisense generation

Hi,

with this pull requiest i try to take the unit testing possibilities of WebEssentials 2013 to a better level.

Things like analysing the FileCodeModel.CodeElements are very hard or impossible to mock. I gave up on that und searched other ways.

I found _VsIdeTestHostContext_ which enabled some testing with visual studio. well it starts and closes visual studio instance, and the run the tests (more an automated integrationtest). It only works with MSTest as far as i know.

The Tests works as following

* a Test calls VSTest.GetTestSolution()
  * this starts Visual Studio (of not allready done in an other test)
  * the Solution TestProjects/TestProjects.sln will be opened (if not allready done in an other test)
  * the Test can now use the Solution to runs its test
* each test which would like to use VS must use the  [HostType("VS IDE")] Attribute

Select the TestSettings in the Test-Menu if would like the Test in the Experimtel hive.

If a test can't load a type of the WebEssentials2013.dll the the WebEssentials Extension must be disabled in the corresponding hive.

I started only with some test, far from complete.

If you merge this PR, than i will complet the Unit Test for the Intellisense generation. And others can also write test.