do not use deprecated compiler module

At least in Python versions >= 2.6, fixes #115
make nose.util.isgenerator() work on Python 3

- True if either a generator expr or a generator function
- isgenerator() to use existing UnboundMethod workaround
Merge pull request #648 from kimscheibel/master

Do not use deprecated compiler module.
Fix docs describing the arguments of formatFailure and formatError

- emphasize that the error tuple has to be returned even if it is not
  modified.
- remove highly misleading note about returning the test arg.
Merge pull request #492 from abeld/formatFailure-doc-fixes

Fix docs describing the arguments of formatFailure and formatError