SQUASHME: docstring, name, get_ipython, refactoring

All of these are just suggestions, can take or not take them.

Summary of changes:
- Import get_ipython before using it to solve some pylint / flake8
  warnings; functionality should be the same as it returns None if
  not in IPython
- Change docstrings to conform to PEP257; should have a one-sentence
  summary, then body, then numpydoc sections.
- Rename UpdateBehavior to ProgressUpdater, and change those subclasses
  to be verbs instead of nouns (e.g., UpdateN instead of MaxNUpdater;
  UpdateEveryT instead of IntervalUpdater)
- Rename CmdProgressBar to TerminalProgressBar
- Moved the wrap_with_update_behavior
  (now wrap_with_progressupdater) to the ProgressTracker instead of
  the Simulator (as other people making ProgressTrackers likely want
  this behavior)
- Made nengo.utils imports relative (will be necessary when #550
  is merged)
- Fixed some references to old classes (e.g. ProgressObserver)