ST_AsX3D opt option for axis ordering and bounding box output

[robe]

Add an opt option similar to what ST_AsGML has to allow outputting coordinates in a different orientation and also just the bounding box of the geometry.

Ideally would have an option to output a true mimimum bounding box (arbitrarily orientated minimum bounding box) not just an axis aligned one. As described here:

 http://en.wikipedia.org/wiki/Minimum_bounding_box

Once I figure out how compute that.