D2 port of xfBuild (alpha version)

[Andrej Mitrovic]

Honestly I don't think xfBuild should be used in IDEs, an IDE already
has all the information about module dependencies and has a better
idea on what to compile.

Currently xfBuild is actually slower than RDMD on full builds (both
the D1 and D2 branch). When I further refactor the codebase I'll see
what is slowing it down. However on incremental builds xfBuild is
faster. It still likes to crash sometime though, so RDMD is safer to
use at the moment.

Anyway RDMD is DMD-specific, but I want xfBuild to work with multiple
compilers (DMD/GDC/LDC/SDC) and linkers.