Add model for Cisco small business switches

Supports 200, 300, 500, and ESW2 series switches
Merge pull request #44 from thetamind/cisco_small_business_switches

Add model for Cisco small business switches
update about Cisco SMB support
Add ExtremeXOS
Add Brocade NOS
Match IP address explicitly

- New method node_want?
- Don't match node to node[:name] if node[:name] is an ip address.
  Otherwise we might match to wrong ip address.
Return true if node_want is nil

- Otherwise all nodes can not be loaded.
Remove ntp clock-period completely

emj complained that it sometimes pops in, sometimes pops out. I guess
really no point having it there.
Fixed small bug in powerconnect enable code
Added model for cisco NXOS