add UncleSam::FederalBracketDefiner

determines federal tax rate from taxable income and filing status
UncleSam::Calculator > UncleSam::TaxableIncomeCalculator
rename FederalBracketDefiner > FederalTaxCalculator
federal tax calculator now calculates the amount like it's suppose to
remove redundant method
add another expectation just to be safe
broke up the single responsibility method a little bit
Merge pull request #2 from PatrickMa/federal_tax_bracket_definer

Federal tax bracket definer