[WIP] Enable/Disable service notifications

Hi, i'm a user of this awesome product, and use for our production systems. 

I'd like to enable/disable service notification, and implemented this PR. 
After merging this, we can enable/disable monitor like this:

```bash
$ curl -X PUT 'http://example.com:4567/_notifications/host01/DISK' -d '{"command":"enable"}'
{"result":true,"object":[{"data":{"host_name":"host01","service_description":"DISK","action":"ENABLE_SVC_NOTIFICATIONS"},"result":true,"messages":[]}]}

$ curl -X PUT 'http://example.com:4567/_notifications/host01/DISK' -d '{"command":"disable"}'
{"result":true,"object":[{"data":{"host_name":"host01","service_description":"DISK","action":"DISABLE_SVC_NOTIFICATIONS"},"result":true,"messages":[]}]}
```

But I'm not sure whether this URI is proper or not. Could you tell me your thought? In addition, if I implement endpoint for other operation (e.g. enable/disable host notification), how should I implement it?

Of cource, I'm ready for editing this PR. Thanks in advance.