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()

This should resolve #244. How to use the VLAN name filter:

```python
import SoftLayer
client = SoftLayer.Client()
network_manager = NetworkManager(client)
network_manager.list_vlans(name='my first vlan')
```

This is added to the list call instead of the detail call because it could potentially return multiple vlans which have the same name. Additionally, the VLAN name is now shown with the `sl vlan list` command