Got rid of Network's available column.

This isn't really needed; the only possible meaning of this is "no
project", which we can check by seeing if the project column is null.
Started working on a Vlan Model.

This is needed to actually implement isolated networks on top of vlans.
Added an `__init__` to haas.model.Vlan.

This fixes some failing tests.
Network/Vlan allocation mostly implemented at the model layer.

There are some failing api tests, which look like it's just that they
aren't making vlans first. I'm sure there are more tests that are worth
writing, but the code is roughly in the right shape now.
Stubbed out haas.api.vlan_register, and wrote some tests.
Implemented api.vlan_register
vlan_register test now expects a more specific error.
Got some tests passing.

We need to create vlans before networks now.
Added two tests, one fails.
Fixed a failing test.

We need to set the vlan back to available when we delete the
corresponding network.
Merge remote-tracking branch 'moc/devel' into vlan-model
Two more tests, based on George's suggestions.
Merge pull request #148 from zenhack/vlan-model

Vlan model (WIP)