Allow wro4j-maven-plugin callers to skip execution

Adds a property to wro4j-maven-plugin to allow skipping execution.
If 'skip' is set to true, Wro4j does not run. This allows users
to skip Wro4j based on a command-line maven option eg for
development.
Merge pull request #154 from akhaku/1.7.x

Allow wro4j-maven-plugin callers to skip execution