Improve the verbosity of error when using iframe.

Ensure to cancel the ongoing promise if queue is cancelled.

This reduces the number of DOM modifications (only one replace).

This also ensure that the element content is empty.

Previous event listener will also be discarded (and so, reduce memory usage).

declareGadget element parameter can now also be a tag name.

This will simplify error catching.

Contains speed optimizations, mostly by reducing the number of promise created.
This reduce the garbage collection load.

No need to execute async code if the gadget class is already loaded.

Reuse Queue created from callback directly.

No need to wait for async result when no acquired method is defined.

Add mutex support in declareMethod.

Example: rJS(window).declareMethod('getFoo', function () {}, {mutex: 'foo'})
It will prevent 2 concurrent execution of the getFoo calls.
The same mutex can be used by other methods if the same name 'foo' is used.

Mutex execution should be cancellable.
callback should always be cancellable without preventing the mutex to be released.

Fix a memory leak.

Seems the .then method never realse the memory if the original promise is kept.
Manually create new promise instead each time.