Change method signature on repository interface

It has more sense to say it return variables names instead
variables, can lead to confusions.
There is no need for this anymore

Contxts for rules will be built with built in features of the PHPExcel
library
Change the building of the context.

Change the building of the context to build it from the variables used in excel file.
Merge branch 'master' of michelrc

At: https://github.com/michelrc/sheet-validator into michelrc-master

Conflicts:
	src/Payroll/PayrollValidator.php
	src/Payroll/Rules/RuleRepository.php
	tests/Payroll/Test/Rules/TruthTable.php