Coding style conventions/guide

We had some discussions on #94 about coding style. @jakirkham suggested we open a specific issue to discuss them.

I have three specific things that I find "inelegant" (yes, I know, it's imprecise) as currently implemented in `dask_image`.

1. fully specified imports, e.g. `dask.array` instead of `da`, `numpy` instead of `np`. Some of these abbreviations are very ingrained in the community and the full specification is surprising to read. (Not to mention annoying to write.)
2. Code definitions in `__init__.py`. Again, this is unconventional. `scikit-image` doesn't use these and still gets a nice API generation from sphinx, so I don't think it's a major hurdle to get sphinx to behave well for a more conventional code structure.
3. The import name with an underscore in it. ;)

I'll throw a bonus idea in here: we should rename `input` in all the ndimage functions. It was a bad choice from SciPy, and they're stuck with it, but I don't think dask-image should be bound by it.

Anyway, whatever is decided with the above points, it should probably be codified somewhere, and preferably it should reference the style guides for bigger projects, so that these aren't "just for dask-image" conventions.