Add runtests.py to make life easier

Add a runtest.py script which builds and runs tests for Scipy correctly just by
```
python runtests.py
```
on the top level dir. It can do also a number of other useful tricks (enough for me to drop my custom makefile).

Also, update the instructions in HACKING.txt accordingly. I also removed the explicit recommendation to virtualenvwrapper --- it takes more than a couple of minutes to install it. It's still mentioned, though, for those interested.