Adding skip parameter to Mojos

This PR adds a skip parameter to the android maven plugin, so it is possible to use the packaging=apk and avoid the execution of goals associated to the default lifecycle.

We use this parameter to reduce the build time (maven clean install) in our project http://github.com/e-ucm/ead/ that is a multiplatform application, and the time related to building the apk was anoying most of our developers (those not dealing specifically with android).