Add agent support

[vincentbernat]

Currently, snimpy only implements a manager. Adding agent support would be nice. At first, constant values could be registered to the agent with the exact same syntax than with the manager :

a = Agent()
a.ifDescr[2] = "eth0"
a.ifDescr[3] = "eth1"
a.ifInOctets[2] = 54548754
a.ifInOctets[3] = 0
a.serve()

Then, we could attach functions. When attached to a table object, they will be provided with the appropriate index.

def randomValue(ifindex):
   if ifindex in range(2,6):
      return random.randint(0, 1 << 32 - 1)
   return None

a = Agent()
a.ifDescr[2] = "eth0"
a.ifDescr[3] = "eth1"
a.ifDescr[4] = "eth2"
a.ifDescr[5] = "eth3"
a.ifInOctets = randomValue
a.ifOutOctets = randomValue

Later, we could add write support too!

[vincentbernat]

And of course, full agent or AgentX support!