Update reference to CircularAperture in poppy.Instrument
Merge pull request #25 from josePhoenix/issue_24

Update reference to CircularAperture in poppy.Instrument
let Instrument class pass through dict or tuple sources to calcPSF; issue #28
Merge branch 'master' of github.com:mperrin/poppy
Docs: Add about.rst, which I had forgotten to add and commit before.