Added ConfigInterface class to encapsulate Interface data
Added ConfigInterface class to encapsulate Interface data

This is intended to be used like so:

```python
In [7]: c.interfaces = [
   ...:     ConfigInterface("public"),
   ...:     ConfigInterface("vlan", label="new", ipam_address="10.0.0.0/24"),
   ...: ]

In [8]: c.save()
Out[8]: True

In [9]: c.interfaces
Out[9]: [Public Interface, Interface new; purpose: vlan; ipam_address: 10.0.0.0/24]
```

This makes it a lot easier to create interfaces, compared to using
dicts.  Dicts are still supported though, although all interfaces will
be rendered into the ConfigInterface class now.

linode/python-linode-api