Cleaned up DNAT Wrangler client code.

https://github.com/dice-cyfronet/atmosphere/issues/59

 - Moved constants with max and min port numbers to app/services/atmosphere/dnat_wrangler.rb
 - Removed unused variable in dnat_client method
 - deleted lib/atmosphere/wrangler.rb
Removed remove_dnat_for_vm from DnatWrangler

https://github.com/dice-cyfronet/atmosphere/issues/59
Merge pull request #62 from dice-cyfronet/wrangler_cleanup

Dnat Wrangler code cleanup with correct parent