Domain validation use cases

This is a list of cases where PublicSuffix.valid? returns true and the
domain is apparently invalid.

- Domain has a scheme :// (GH-15, GH-33)
- Domain contains spaces (GH-52)
- Domain contains double (GH-25, GH-36)

This is an hard decision to make, but as I explained more than one, the
purpose of this library is not to formally validate the syntax of a
domain or an URI.

The library expects the input to be a domain, and can't run all the
possible guesses and checks to guarantee the input. This is a
constraint that the caller must ensure. Entering the business of
validating URIs would make the maintenance of this library very
complicated.

This is an example of a simple validation that could be used before
submitting the string for tokenisation.

```ruby
def valid_uri?(name)
  uri = URI.parse(name)
  uri.host != nil
rescue
  false
end
```

I don't exclude to add such filter in the future in the `#valid?`
method, may be as wrapper for the current `valid?` behaviour. But for
the time being, this library is not intended to formally validate URIs
or domains.

(closes GH-25, closes GH-36, closes GH-52)