Cleaned up some warnings, added gitignore

i am too unmotivated to do real coding >_>
Fixed bugs, added rudimentary skill system.

Bugs fixed include a bug where buying a new ship increased your money, a
bug introduced by the skill system wherin buying 100 of anything
infinitely increased your money, and a bug where AI ships randomly crashed
the game due to div/0.

Skill system currently has exponential XP curve (1.5^x) and provides the
bonus of a 1% discount on buys and 1% bonus on sells per level.

Need to implement item buy throttling.