Report 500 status via an APIError.

Previously, node_power_cycle was the only api call making use of the
ability to return a status code. This patch defines an APIException to
use for that purpose, and removes the ability to return a status, which
simplifies the request handler a bit.
Add unit tests for haas.http.

There are some infrastructural classes in here that should probably get
moved to test_common or the like; in particular I wrote
HTTPEquivalenceTest with exercising the real api functions in mind.
Expose nodes' mac addresses in show_node.

This will be useful for users who wish to use the mac address to
distinguish nodes when provisioning them.
Fixed erroneous calls to network_create in deployment tests

These were overlooked when Fancy Networks was merged (#275).
Document deployment test failure causation

Add doccumentation to the deployment tests explaining why the headnode_create
test fails.
Set 'dry_run' in simple vlan tests
Merge pull request #283 from zenhack/expose_node_macaddrs

Expose node macaddrs
Merge pull request #285 from ryabouza/fix-network-create-calls

Fixed erroneous calls to network_create in deployment tests
Merge pull request #286 from ryabouza/document-deployment-test-issue

Document deployment test failure causation
Merge pull request #284 from gsilvis/fix-simple-vlan

Set 'dry_run' in simple vlan tests
Merge pull request #281 from zenhack/http-tests

Http tests
Update url field in setup.py.

Looks like this was never changed when we renamed the repo.
Merge pull request #287 from zenhack/setup.py-url

Update url field in setup.py.
Implement and lightly test headnode_delete
Test headnode deletion more

Test that deleting a running headnode works correctly.
Merge pull request #288 from gsilvis/fix-headnode-delete2

Implement and lightly test headnode_delete
Implement null auth backend

Authentication/authorization backends belong in haas/auth_mechanisms .  It's
possible that the API of that plugin will have to be expanded to cover what
the CLI needs to do.