Added better isDirty support

- relates to commit https://github.com/MrOnion/maven-git-commit-id-plugin/commit/80d4afd44be3adf369816257c5e4190a6d733776
Merge branch 'santeriv-patch-2'
No need to test Repository#isDirty() in GitCommitMojoTest
Renamed dirtyCheckLoose to dirtyIgnoreUntracked
Reuse JGit's Status object in JGitRepository#isDirty
Updated description of the dirtyIgnoreUntracked property
Fixed typo in description of gitDir property

Congratulations to my 2 year old son for his first OSS contribution.
Move default values into property definition

This improves goal documentation.
Added more info on GitInfoClassMojo.outputDirectory