MultiSite livestatus support

* New class: pynag.Parsers.MultiSite
* Behaves just like pynag.Parsers.mk_livestatus
* Should make MultiSite support in Adagios easier

Old way of writing livestatus code:
```
l = mk_livestatus(livestatus_socket_path="...")
l.get_hosts()
```

New way:
```
l = MultiSite()
l.add_backend(path="...", name="local connection"
l.add_backend(path="...", name="remote connection")
l.get_hosts()
l.get_hosts(backend="local connection")
```

Comments much appreciated.