Changed sim.copied to sim.get

This simplified some example syntax, and is done for the ref simulator in [nengo/#163](https://github.com/ctn-waterloo/nengo/pull/163).

Should things like this be part of some base class that all simulators inherit from?

I also converted some unit tests to use the `unittest` module while I was at it; but, I'm unable to get all of the tests to pass. I get
```
Ran 111 tests in 186.108s
FAILED (failures=3, errors=35)
```
though, before this change, I got
```
Ran 101 tests in 193.515s
FAILED (failures=1, errors=27)
```
so the additional errors are likely tests that weren't running before. Not sure if this is due to my setup or old tests.