Bug fix for #531 with extra features

This pull request deals with bug id #531 - a request for object dependent SNMP configurations - I've done this by adding a new attribute 'SNMP Conn', mapped to the 4 object_types that the snmp sync functions work for.

In addition, I've added the ability for attributes to be hidden from the object summary page - while still being visible and alterable from the properties tab.

And the final change is to the SNMP Sync tab now only displays one form with the various fields being displayed dependent on the SNMP version, (and in the case of version 3, security level).