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?
@larrybradley - in general I agree that if you have many functions as before, it makes sense to try and re-use the docstrings, but for just two functions I don't think it's worth it.

Similarly, I would suggest writing out the percentile parameters since we know what those are - I'd only use ``**kwargs`` if I was passing parameters to e.g. a matplotlib function, which typically take dozens of arguments.