Add skip option to maven configuration.

It enables to skip the target. It can be useful on Travis CI for instance to install all modules's artifact and bypassing tests, coverage and coveralls. (At least I need it for a lib of mine :) )
Complete the feature by adding a property to job

Sorry, I only used github "IDE" online, that's why there are 2 commits.
#19, #20: Cleaned skip property implementation.