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.
Not really; my thought has always been that `get_` is usually redundant and if you get rid of it not much changes.

That said, we do it in several places:
```bash
$ grep -RIic "def get_" nengo | grep -v :0
nengo/tests/test_solvers.py:4
nengo/config.py:2
nengo/builder/connection.py:1
nengo/builder/network.py:1
nengo/cache.py:6
nengo/spa/spa.py:8
nengo/spa/pointer.py:1
nengo/spa/thalamus.py:1
```