Initial work on Linux port.
Linux: Fix input event loop.
Linux: Cleanup of Makefile and some warnings.
Linux: Change indentation to spaces.
Linux: Fixed glitch in mouse event handling.

IO_PushEvent sometimes pushed an unitialised event, causing the ship to
shoot from the very beginning of the level.
Linux: Added a README with build instructions.
Merge pull request #1 from xevz/master

Linux port using SDL