Build separate libraries for VSGI implementations

[arteymix]

Each VSGI implementation now has its own shared library.

• implementation can be loaded with gmodule-2.0 and GLib.TypeModule
• only the FastCGI implementation has to deal with fcgi.vapi
• provide a VSGI.Mock implementation for testing purposes

I had to rename VSGI.Soup to VSGI.HTTP because it was conflicting with Soup namespace. It's a much better naming anyway.

Used implementation has to be explicitly stated in the build:

valac --pkg=valum --pkg=vsgi-http src/app.vala

[arteymix]

Everything's good, even the coverage ;)