Created interface for all Items, as well as added in the abstract class
from the last iteration of the DynamicMarket plugin. (NOTE: May need to
rework some of that, but for now we'll just copy it)

Added in a class to represent a transaction between a player and the
market, as well as created enums to represent transaction type and
status. (NOTE: May need to rework this also, not sure how I feel about
it)
Merge remote-tracking branch 'origin/master'