URL's missing a scheme fail silently

If I create a furl object without a scheme like so:

```
>>>from furl import furl
>>>x = furl('www.google.com')
```

It fails in weird ways that aren't exactly intuitive.

I would expect that furl should either raise an error here or create a URL without a scheme.

Instead I get weird results like:
```
>>> g.origin
'://'
>>> g.host
>>> g.netloc
```