Turn *_connections() constants into enums

[giampaolo]

This is similar to #469 and it aims to turn connection-related types and families into enums on Python 3.4 in order to have a more readable repr()esentation. The functions affected are psutil.net_connections() and psutil.Process.connections(). Later on we'll have to do the same thing for net_if_addrs (see #376).