New API method: - [ ] `grappa.when` ```python if grappa.when(should.be.os('centos')): 'nginx' | should.be.running 80 | should.be.listening 'eth0' | should.have.interface if not grappa.when(should.be.os('ubuntu')): 'apache' | should.be.running 8080 | should.be.listening if '/opt' | grappa.when(should.path.exists): '/opt/foo' | should.directory.exists '/opt' | should.have.subdirectories(['log', 'app', 'run']) ```