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.

This will make JS stack more readable.

/reviewed-on nexedi/renderjs!12

/reviewed-on !11

Iframe: ensure all channels are OK before calling ready

Do not update jslint version which deprecates indentation options.

Add onLoop service.
Allow manualBootstrap.

The inital loaded HTML does not contain anything of renderjs and gets added only after pageload.
Instead of registering init() for the DOMContentLoaded event directly, a deferred promise is used which gets resolve either by the event, or by a manual call of rJS.manualBootstrap().

Tests:
* injects rsvp/renderjs in a blank iframe
* makes sure renderjs is only present after it was injected
* calls manual bootstrap to initialzed renderjs
* creates small test gadget to ensure renderjs is working within the iframe (after bootstrap was executed)

onStateChanges bug fixes

Nearly drop iframe timeout.
Add reportGadgetDeclarationError acquired method.

This can be used to catched loading errors from HTML gadget declaration.
This will prevent the parent gadget to fail.

This should improve renderJS usage on slow network.

Performance improvements.

Allow browser to fetch the JS files in parallel.
Loading should stay synchronous.

Speed up loading the declarative sub gadget definition.

Use document fragment to add all dependencies with one DOM call only.

Do not create a DOM document to calculate an URL.

Resetting the internal gadget state leads to unexpecting errors, and so,
it was a bad idea.

In case of onStateChange cancellation/error, reset the internal gadget state.
It will simplify the code of each onStateChange function, as developpers will
not have to handle such issue.