Replace OIS with SDL2.
Change input API to use SDL2 enums.
Add override keyword to MusicStream classes.
Merge branch 'dev' of https://github.com/Ponykart/PonyKart into dev
Moved OpenAL extensions into ponykart++.
Fix typo.
Convert window/context creation and main game loop to use SDL2.
Merge pull request #12 from Destineer/dev

Finished SDL2 conversion. Will NOT build correctly on Windows.