Fixed bug with async xflow operators.
Fix for issue #46. Correctly use an object map for opaque renderObjects.
Changes order in which asset data is overwritten.

When extending an assetdata or assetmesh, the data linked via
"includes" will override the data of the original entry.
Adapted asset pick filter to support filtering via classNames + test.
First steps towards assets within assets.
Several small fixes - first tests are working.
Several fixes for nested assets + handling of _postTransform assetdata entry.
Added new xflow.get operator used to extract array values by index.
Added match attribute to assetmesh.

The match attribute allows to override multiple assetmesh elements at the same time.
Adapted asset tests.
Started with tests for nested assets.
Finished tests for nested assets.
Merge branch 'instance-improved' into develop
Optimized attribute handler implementations.

- Attribute Handlers are now independent of elements
- Attribute Handlers are constructed only once per element type
- Data is stored in separate "storage" field in ElementHandler
Write element interface into prototype - much memory saved!
Change element implementation to use MutationObserver instead of (deprecated) MutationEvents.

- Added Method XML3D._flushDOMContent that will immediatly flush all stacked DOM changes, updating internal content
- All read-access methods should automatically call XML3D._flushDOMContent to garantuee correct output
Updates several test cases to work with new MutationObserver implementation.
Added load events as well as 'complete' and 'texComplete' property for all DataContainer elements

+ Tests
Changed behaviour of texComplete and textureload event

- texComplete is always false if complete is falls
- textureload event is fired whenever the data and all its textures are loaded (even if there are no textures)
- Extended test cases
Removed texComplete and added more general progressLevel

- progressLevel indicates how much of the data has been loaded
-- progressLevel == 0 => Still loading referered data elements
-- progressLevel == 1 => Still loading textures
-- progressLevel == Infinity => everything has been loaded
- This is currently supported for all data container
- Assets and model follow next
- In the future we can assign a "loadLevel" e.g. to data elements
-- content is loaded in order of increasing loadLevel (loadLevel=0 is loaded first)
-- All data element are by default loadLevel=0
-- All textures are by default loadLevel=1
-- if progressLevel > loadLevel, than this content has been fully loaded
- the load event is called when progressLevel reached "Infinity"
- A progress event is fired whenever the progressLevel increases