Initial commit of libSDL-d sources

Slightly modified version of modules written by Jonathan Marler
with some elements based on Nick Sabalausky "SDLang-D" project

Changes from original published work-in-progress sources:
- added basic dub project layout
- renamed package name to libsdl
- removed most redundant version identifiers

Commiting this in state where both `dub test` and `dub build :cli` finish with no errors