Use the proper HTTP method when saving

The previous implementation of `Repository.save` was destructive and had
the nasty side-effect of wiping all of a repository’s content when it’s 
configuration was updated! For more on this WTF see:

http://www.jfrog.com/confluence/display/RTF/Artifactory+REST+API#ArtifactoryRESTAPI-CreateorReplaceRepositoryConfiguration

TODO:

* [ ] Apply behavior to `Artifactory::Resource::Group.save`
* [ ] Apply behavior to `Artifactory::Resource::User.save`

/cc @opscode/release-engineers 
Are we using `existing` here? I don't think we should save it to a variable if we aren't. I also think I wrote an `exists?` method, but I could totally be making that up.