components: preserve them over import and export.

UFO component handling.

This is aimed at part 1 of the comment in issue 298:
https://github.com/metapolator/metapolator/issues/298#issuecomment-63084752
    "roundtrip: make sure that components from imports end up in the skeleton layer. Make sure that components from the skeleton layer end up in the exported UFO. No need to detect circular dependencies here."

Specifically I have tested with the following and the chain of component references is preserved in mpout.ufo from 
the components.ufo sample file.

    $ alias mp=./metapolator
    $ rm -rf /tmp/mptest; mp init /tmp/mptest
    $ mp import ./data/fonts/test/components.ufo  /tmp/mptest/components2
    $ mp export /tmp/mptest/components2 /tmp/mpout.ufo


ok, looking over the part workflow it seems that the export function is to get the transform from the CPS instead of directly from the MOM. So the CPS can update the transform and leave the MOM intact as it always was. So it doesn't look like setters are going to be needed for these in MOM and so I'll trim them off as above. 

I'm still wrapping my head around how the MOM and CPS currently interact.