Added htmlStatement

Added method htmlStatement() to the Customer class.
Moved getCharge() to Movie

Moved the function getCharge() from Rental to Movie.
Moved getFrequentRentalPoints() to Movie

Moved the method getFrequentRentalPoints() from Rental to Movie.
Added class Price and subclasses

Added the abstract class Price and its three subclasses RegularPrice,
ChildrensPrice and NewReleasePrice.