Fixing up import statements

Removing/commenting unused imports,
and avoiding `from x import *` usages.
Use new-style classes
Raise NotImplementedError for abstract base class
Avoid using backticks for repr()

They're deprecated, and they're strange to look at.
Use `not in`
miscellaneous style fixes
better format strings
Merge pull request #1 from all-umass/style_fixes

Lots of style fixes