[WIP] Monitoring fixes

While trying to work with monitoring, I couldn't get any of the example code to run because access to the response object has completely changed.

```
In [8]:   entity = cm.create_entity(name="somehost.domain.com",
   ...:                             ip_addresses={"example": "192.168.1.1"})
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-67b5b1d6fafa> in <module>()
      1 entity = cm.create_entity(name="somehost.domain.com",
----> 2                           ip_addresses={"example": "192.168.1.1"})

/usr/local/lib/python2.7/site-packages/pyrax/cloudmonitoring.pyc in create_entity(self, label, name, agent, ip_addresses, metadata)
    949                 ip_addresses=ip_addresses, metadata=metadata,
    950                 return_response=True)
--> 951         status = resp["status"]
    952         if status == "201":
    953             ent_id = resp["x-object-id"]

TypeError: 'Response' object has no attribute '__getitem__'
```

What's included here is *only* a fix for create_entity. It looks like the entire module needs fixing. Let me know what you think and I'll tackle the rest.