Add SDL2 - going to convert to this from SFML
Move reference material to Dropbox instead
Build and link SDL2 alongside
Remove SFML and stub out anything that doesn't work, Ryan Gordon style.
Get window up
SDL2 relative mouse mode doesn't behave correctly on Linux, so implement it as I was when using SFML.
Fix image loading
Start adding in JPL data
WIP combine the 'Moon' and 'Planet' types, and add a non-SSE vector type