entity using other local reference frame than ENU?

Hello,
after seeing this [post](https://groups.google.com/forum/#!searchin/cesium-dev/enu|sort:date/cesium-dev/WiI0xn7BgBI/SBocZp_ADgAJ) and after seeing that ``entity.computeModelMatrix`` is now public (see [here](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/Entity.js#L596-L618)), do you think it's a good feature to make the the default ``Transforms.eastNorthUpToFixedFrame`` in line [613](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/Entity.js#L613) more versatile and use an other transform?

Start with Source/DataSources/Entity.js, especially entity.computeModelMatrix and the Transforms.eastNorthUpToFixedFrame call around line 613. Read the linked discussion to determine which alternative local reference frames are needed and how callers would select them. Done means the supported transform behavior and its API are defined and validated by tests.

Written by the indexing model from the issue text.