Organize / Freshen Solution Structure for Updating

[jeroldhaas]

Solution to be structured into separate projects, one for each MVC version, so that test code can be shared among them.

[jeroldhaas]

Razor official test cases available in the Razor repo. Might be good to nick those for conversion to agreed test framework.

[jeroldhaas]

@dahlbyk @DerKnerd please have a look at my reorganization branch and let me know if there's anything that stands out as needs fixed.

I'd like to reduce the amount of solution restructuring once we get test code and actual library code back in active development. Measure twice, cut once. 😉