Refactor scale_image module

This PR refactors the `scale_image` module, including combining the `scale_*` functions into a single function (https://github.com/astropy/imageutils/pull/11#issuecomment-51109051, https://github.com/astropy/imageutils/issues/16).

@astrofrog  I have no equivalent to APLpy's `vmid` keyword for `log` scaling (`log` scaling in APLpy is a bit different).  For `asinh` scaling, `noise_level` here is equivalent to `vmid` (but the defaults are not the same).

Can someone review this?