RFR: IO cleanup

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