Mostly non-functional cleanup for PEP8, typos, add missing doc, etc.

Also update setup.py so that it is an executable, and that
`./setup.py test` runs unittests.
Add from_world/to_world as discussed in #6
Move methods off class to module, and rename to loadsw/dumpsw

Also translate to/from center- and corner-based coordinates.
fix testing on errors
Merge pull request #7 from mwtoews/world-file

Add from/to world file support