Read/write world files

[mwtoews]

As discussed several months ago on rasterio#80, I thought it would be convenient to read/write world files, which use the same 6 coefficients as Affine.

Any ideas for names, prototypes, behaviour? The reader will certainly be another @classmethod that returns an Affine object. Initially, I was thinking from_worldfile(fp), but it could also be load_worldfile(fp) and/or loads_worldfile(s) for reading. Similarly, dump_worldfile(fp) and/or dumps_worldfile() for writing.

[sgillies]

Sounds good to me. I like from_world() and to_world() for functions that take and return sequences, but I'm 👎 on having any file I/O methods here in affine.