[MOJO-1945] Add tidy:check goal.

Fail the build if the POM is not tidy. Output a request that tidy:pom
should be executed.

git-svn-id: https://svn.codehaus.org/mojo/trunk/mojo/tidy-maven-plugin@19842 52ab4f32-60fc-0310-b215-8acea882cd1b