Adds VLAN name filter

Also refactors the network manager quite a bit. Here's a list of notable changes:

 * list_subnets() no longer uses a client-side filter
 * removes un-needed "mask[]" to mask strings
 * Removes superfluous IdentifierMixin
 * Re-organized add_subnet() and added doc string
 * Removed code duplication for edit_rwhois()