Feature netcf

This PR removes compact framework 2.0 support and fixes up the netcf 3.5 project so it basically works. There are currently two failing tests under CF 3.5, both related to DelayedConstraint. I'm merging this into master anyway, for the following reasons:

1. It doesn't impact our CI build or our other supported platforms
2. It removes something we have decided not to support (CF 2.0)
3. It advances the CF 3.5 build from not building at all to having only two failing tests
4. Somebody else is already working on the DelayedConstraint issues.

We will continue to maintain CF 3.5 in the alpha releases. We have a potential committer who wants to work on it and in that case we'll continue to support it. In any case, we don't want to kill the code until we decide whether to add CF 3.9 to our main solution, since they are source-compatible.