Release 0.2.2

Minor improvements.

• Removed excessive synchronization from Promise. In particular, it no longer executes alien code (then callbacks) within its synchronized blocks, as this is fragile and deadlock-prone (Effective Java item 67). Locks are now only held strictly as necessary.
• Reject callbacks are now declared as throws Throwable rather than throws Exception. This makes the "execute cleanup code and rethrow" idiom more natural to express in thenApply and thenAccept:

promise.thenApply(result -> { ... }, exception -> {
    loadingIndicator.hide();
    submitButton.enable();
    throw exception;
});