Renamed each in amountFor(...)

Renamed each in amountFor(Rental each) to aRental.
Moved and renamed amountFor(Rental aRental)

Moved amountFor(...) from Customer to Rental and changed name to
getCharge()
Removed redundant thisAmount
Moved frequentRenterPoints calculation

Moved calculation of frequentRenterPoints to new method
getFrequentRenterPoints() and moved this method to the Rental class.
Replaced totalAmount and frequentRenterPoints with own methods

Replaced totalAmount with new method "getTotalCharge()" and
frequentRenterPoints with "getTotalFrequentRenterPoints()"