Adds ability to list VLANs by name. Some refactoring of network manager.
Cleans up NetworkManager.add_subnet slightly
Removes string conversion from loop
Subnet and Global IP commands now use resolve_id in order to ensure there's exactly one matching id
Fixes refactor bug
Renames vlan_name argument to just name with list_vlans()
Fixes incorrect doc
Merge pull request #246 from sudorandom/issue-244

Adds VLAN name filter