coordinate: add support for classifiers

Allow one to specify a classifier as part of the coordinates. This is especially
useful to refer to test jars for example.

From http://maven.apache.org/pom.html#Maven_Coordinates:

classifier: You may occasionally find a fifth element on the coordinate,
and that is the classifier.
[...] Those kinds of projects are displayed as groupId:artifactId:packaging:classifier:version.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>