- Updated docs to reflect changes in 2.4.

  - Reorganized the documentation
  - Shifted focus a little more towards "literate testing"
  - Documented new functions and classes:
    - testfile()
    - Example, DocTest
    - DocTestParser, DocTestFinder, DocTestRunner, OutputChecker
    - DocFileSuite
    - DebugRunner, DocTestFailure, UnexpectedException
    - register_optionflag()