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.


Testing done: verified that the following Jarfile would be parsed correctly with JBundler:

    jar 'com.ning.billing:killbill-api', '0.1.48'
    jar 'com.ning.billing:killbill-util:tests', '0.1.48'

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>