Added ability to set context name for SNMP variables

Hi Pieter,

I've added the ability to set a non default context name for SNMP variables.

One change which may be a bit more controversial is my use of the _objs_ attribute:  `agent._objs[context][oidstr]` which could break someone's code, but I couldn't think of a neater way to do it. `agent.getRegistered()['']` gives same behaviour as before.