Support for Netbox IPAM/DCIM

It would be cool if observium gets a netbox integration.

Netbox is an IPAM and DCIM.

https://github.com/netbox-community/netbox

So i decided to include some quick and dirty code to display a navbar tab with a link to my netbox instance.

You also need some config-variables:
// Netbox
$config['netbox_enable'] = 1;
$config['netbox_api_token'] = "xxxxxxxx";
$config['netbox_api_base_url'] = "http://netbox.xxxxx.xxxx/api/";

I attached a patch. You can use and implement it if you find this cool, too!

I don't know how to handle the icons - so i used the "info" icon which is not cool. And i did not put much effort into exception catching, because i dont know what you think about it.

You could also implement other things, like a sub page with details requested via the API, so that you don't need to leave the observium page to get all those information.