Fail if $version is not set appropriately.
remove defaults from nexus::package.  This is meant to be called from
nexus so don't bother.
Add ability ton control the name of the directory under $nexus_root that
nexus will live in.
don't create $nexus_root.  That could be a system directory (/srv,
/usr/local) and we have no bussiness trying to alter that.  Leave it's
creation up to the caller.
Add ability to pass user and group name to class.  Needed so we can
change ownership of extracted directories.
We're no longer ensuring that $nexus_root exists so don't require it.
Explicitly use $nexus::params::nexus_work_dir
Add service handling.