Initial commit of libSDL-d sources

Slightly modified version of SDLang project by Nick Sabalausky

Changes from original published 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
Fix deprecation warnings from Duration
Accept single-line comments starting in the middle

Previously lexer would consume new line symbol too resulting in
broken parsing. Most generic fix is to never consume new line symbol after
a single-line comment as it does no harm for parser anyway.
Add sample.sdl from SDLang-D project to tests