Fancy progress

This PR implements fancy progress tracking:

* Progress bar will automatically be shown if the ETA is more than a second.
* Progress will be displayed visually as a bar and also as a percentage number.
* The progress bar displays the ETA (estimated time of arrival). (Though, I'm using the simplest possible formula to calculate it right now.)
* The progress bar will display how long the simulation took after finishing.
* The width of the progress bar adjusts to the terminal width if Python >= 3.3 is used (otherwise it defaults to 80 characters)
* In an IPython notebook a fancy HTML/Javascript progress bar will be rendered.

This PR is an alternative suggestion to PR #476.
Is this really Nengo related? I don't think we should add a function if it is not used by Nengo itself or something you would need Nengo for.
Also, do you want to know whether you are in interactive session *and an IPython interpreter* or just that you are in an interactive session (which might be bare CPython)?