Implemented parallel execution of targets in the project

Hi Ryan,

I've added a feature similar to 'make -j' to NAnt - thought it is a bit unfair to have a single-threaded implementation when most of us use multi-core systems.

The implementation passes the same set of tests as the original implementation on Mono/Linux, and now I need your help and opinion on integrating this into NAnt core.

With best regards,
Dmitry