Add copy support

Add support for the copy API method. This enables things like this:

require 'cobbler'
Cobbler::Base.hostname = 'cobbler.example.com'
Cobbler::Base.username = 'me'
Cobbler::Base.password = 'pass'
Cobbler::Base.debug_enabled=true
Cobbler::System.debug_enabled=true
a=Cobbler::System.find_one('testme.example.com')
a.copy('testme2.example.com')