Add Psr/Log library

We are going to need this in near future to report back
evaluation results.
Add more methods to RuleRepository contract

Add methods for dependency handling of logger and rule builder, just
enforcing setting them out. Also declared from now on that Rule
Repositories must be named.
There is no need for this anymore

Contxts for rules will be built with built in features of the PHPExcel
library
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
Add support to external logger.

Logger must be PSR-3 compliant, it will be used on rules evaluation.