Removed the vibe.d dependency by dragging in a minimal set of interfaces of vibe.d ...

The interfaces are reimplemented using standard Phobos functionality. Calls to
these interfaces will be gradually replaced by direct Phobos calls.

Added rudimentary support for the --arch= option and moved build type flag generation to a common place.

Fixed up the VisualD and Mono-D generators and changed their names on the command line to lower-case.

"dub build" and "dub run" are now also executed using the generator system (RdmdGenerator).

Implemented basic multi-compiler support (with only a dmd implementation right now).

The build settings for the default configuration are now emitted for the MonoD generator.

Finished basic MonoD generator support (compiler/linker settings are not yet emitted).

Dub builds with VisualD with latest vibe. BuildPlatform is passed from app.d to the generators. System libraries are suffixed with '.lib' for VisualD.

Renamed Application to Project and put it into its own module + added a stub MonoD generator.

Initial commit (more or less the same code as used for vibe.d's vpm).