Correct gold check comparision in recall cost

By correcting the assignment of the cost of the unit it now correctly
says whether or not the play actually has the gold to recall a unit, as
after this check it is assumed they do by the program which creates
the possibility of recalling when you have no gold, or negative gold.

Added comments so that the magic number is explained.
Merge pull request #130 from Aishiko/Add-unit-instance-recall-costs

Correct gold check comparision in recall dialog