Decide module layout for new imageops functions

[theotherphil]

[ripytide]

Two approaches I can think of:

1. Put all rotation, flipping, overlaying, cropping and tiling functions all in one module called something like compose
2. Put rotation functions in a rotate module, flipping functions in a flip module etc.. all of which are top-level modules

I think 2. seems easier to work with from a version control standpoint.