expose InsertCoin and InsertCredit to Lua

This commit exposes InsertCoin() and InsertCredit() to Lua under GAMESTATE.  It adds a check to ensure overall credits don't become negative when deducting and only plays sounds if credits are being added.

Themers are instructed to deduct credits by passing negative values to InsertCoin.
update InsertCoin and InsertCredit functions

These should now agree with kyzentun's suggestions.
touch up style to InsertCredit and InsertCoin

Third time's the charm.
Added djpohly to credits.