Setup for Python 2/3

This will almost certainly still have some problems, but it's at least working for me now.

These changes:
- Give us scripts called `ipython` and (`ipython2` or `ipython3`) for any installation.
- Add a new `setup.py symlink` target, to use instead of `develop`, which installs scripts and symlinks the library into site-packages
- Removes the static script entry points - all our entry points are now automatically generated.