RFR: IO cleanup

[Stéfan van der Walt]

Hi all,

At the SciPy sprint (ages ago), Chris suggested that we clean up the
image loading API.  The following changes simplify `imread` by
removing the `dtype` argument.  This allows for simpler plugins.
Because `as_grey` is such a commonly used parameter, it is left in
place, but now works differently: after the image is loaded, the (new)
rgb2grey is simply performed on it.

Please have a look at

https://github.com/stefanv/scikits.image/compare/master...io_cleanup

Regards
Stéfan