More upgrade plan stuff.
Added some perks to upgrade plan.
State uses a DisplayObjectContainer instead of an array to keep track of DisplayObjects
Menu now has a generated prevMenu and cachedState based on prevState.
TownMenu makes its cachedState invisible.
Added exclusion areas for ui input. Implements #36

Any additional ui objects should hcreate exclusion areas like buttons.
Update README.md
Updated upgrades; added responsed + ideas