Create Logger class for Pacto (#20)

Logger class is just a wrapper around built in ruby logger. This way we
can replace the logger library on the future (if needed)
Use Pacto::Logger on erb_preprocessor
Configure the logger thru Pacto::Configuration
Merge branch 'master' of github.com:thoughtworks/pacto into improve_logging

Conflicts:
	lib/pacto.rb
	spec/unit/pacto/pacto_spec.rb
Merge pull request #29 from thoughtworks/improve_logging

Improve logging