Use Promises to encapsulate download work

Fixes #39.
 - Adds Squeak-Trunk back into build matrix, removes Squeak-4.4
 - Integration Tests (relativeURLs and submitForm) require no Delay but wait directly for the background downloads to complete
 - Replaces those Scamper methods that invoke background processes or that rely on such methods by methods returning a promise (e.g. jumpToAbsoluteUrl --> jumpToAbsoluteUrlAsync)
 - If the sender wants to wait for the promise to complete or to attach a continuation, he can use the return value, otherwise ignore it