Use protobuild for cross-platform builds.

[JeroMiya]

Protobuild is a cross-platform project/solution generator for .Net. It is stable, active, and used by several high profile .Net projects (MonoGame, for one):
http://protobuild.org/

Using it might help to solve this issue:
#28

Notable unknowns:

• There is a <NativeBinary> reference element to include native libraries. It can be made platform specific (Windows, Android, Linux, Mac), but can it be made CPU architecture specific (Windows32, Windows64, etc...)?
• Protobuild has a cross-platform package management system, but can we make our "package" only include the main library and not all of the samples?
• Can we make the samples a submodule of the main module without moving them to another repository?
• Unity isn't one of the listed "platforms" - I'm guessing we count the .net 2.0 windows build as the unity build?
• What about unit test projects? Can it generate unit test projects?