Add a "VibeDefaultMain" version to the package description of "dub init xx vibe.d".

This will allow to invert the current VibeCustomMain version at some point to make vibe.d play nicer as a simple library.
Enforce build requirements when generating VisualD projects. Fixes #188.
Implements "add-local" with optional version argument.
Remove git process call.
Local packages version get re-parsed on PackageManager.refresh(). Not sure what happens if the user specified some other version when adding the local package.