Allow Request to be extended by changing class name 'Request' to static

[bluehaoran]

Change:
$request = new Request(...)
to
$request = new static(...)

This allows Request to be extended. Why might you want to do that? Personally, I needed to add HMAC signing, so I wanted to be able to do this:
$response = HttpfulRequest::get($url) ->sign($key, $secret) ->send();

[bluehaoran]

Closed in favour of #231, which targets Dev instead of Master.