Add Browser#known? method

This method returns true if the browser has successfully been detected i.e.
it has been possible to determine which browser the User-Agent string belongs
to (Chrome, Firefox, etc). If the browser has not been detected then this
method will return false.

This is essentially just some sugar, I think it leads to more readable code in
some situations, e.g.

```ruby
if browser.id != :other
  # have a party, we know the browser!
end
```

vs

```ruby
if browser.known?
  # have a party, we know the browser!
end
```