Allow artifact_file to handle snapshot and latest version correctly

issue #115

This patch allows ```artifact_file``` corectly downloading *-SNAPSHOT and LATEST version :
* It detects artifact version based on existing artifact class method (Thanks to @gregsymons)
* In case of "rolling version", it retrives artifact filename from Nexus metadata and merge it with provided destination folder.
* Cast a log info because provided filename is now ignored when asking for a snapshot version

Anyway, we are now able to download the artifact properly but we still do not know its name. I think it's still very interesting to correct the issue #112