Vpretty

CC @sympy/mechanics. I had to punt on the ASCII printer for the sympy.vector module, because it was using some awful hacks and I wasn't sure how it was supposed to work. So now it just raises NotImplementedError. Anyway this removes all test failures and so fixes https://github.com/sympy/sympy/issues/7679. 

To test this on Unix, set `LANG=C`, like

```
LANG=C bin/test print
```

On Windows, this already happens, at least for me using the cmd shell. 

I plan to add the above to the Travis tests.