Rename a file

awkward.java -> improved.java
Extract statement method

Calculation of thisAmount in statement method is moved into a new method
"amountFor". In addition, .gitignore is updated not to stage .swp files.
Rename variables

Two variables in amountFor method are renamed: each > aRental,
thisAmount > result.
Relocate method

amountFor method is moved into Rental class. In Rental class, the method
is named as getCharge.
Replace a temp variable

A temp variable "thisAmount" is removed and replaced with inquery
"each.getCharge()"
Extract method

Calculation of frequentRenterPoints is done by getFrequentRenterPoints
method in Rental class.