Use Maven co-ordinates and add classifier support

As discussed in PR https://github.com/RiotGames/nexus_cli/pull/63, this PR changes the following items:

* Change terminology from "artifact" to "coordinates"
* Re-order the coordinates to follow [maven conventions](http://maven.apache.org/pom.html#Maven_Coordinates)
* Add support for classifier

So now, valid values for coordinates are:

`groupId:artifactId:version`
`groupId:artifactId:packaging:version`
`groupId:artifactId:packaging:classifier:version`

I created an Artifact class to help DRY up the string splitting and array exploding that was spread throughout the code.  Most of the change in this PR revolves around that class.

I updated the specs to test classifier but not the cucumber tests. Let me know if you'd like me to update the cucumber tests. I was thinking it would be sufficient to have the cucumber always use classifier?