cancel function is missing on promises.

[tolgaulas]

Description
the cancel function is missing, causing programs to hang even an exception is thrown. Despite Promises/A+ does not mention .cancel, many promise capable libraries (i.e GUZZLE, REACTPHP) provides such feature.

Example
$promise.cancel();

[dbu]

to make things optional, we could add a CancellablePromise interface and implement that in the guzzle adapter promise to expose the guzzle promise cancel method.