Right now if you specify a port via `-p $PORT` and via a `config/puma.rb` then puma will incorrectly try to bind to the same port twice and will fail.
This PR calls `uniq!` on the array to remove duplicate ports from the `binds` array.
Add a lowlevel_error_handler, so we can customize the default error
message.
example:
```
lowlevel_error_handler do
[302, {'Content-Type' => 'text', 'Location' => 'foo.html'}, ['302 found']]
end
```
[fix#458]
