Rework setup to allow installing on Python 2 and 3.

Scripts named ipython and ipython[23], etc.

Neither distutils nor setuptools made this easy.
Remove manually written scripts
Add comment to automatically generated scripts.
Merge pull request #4473 from takluyver/setup23

update script generation in setup.py

These changes:
- Give us scripts called `ipython` and (`ipython2` or `ipython3`) for any installation. (and likewise for each of iptest, ipcontroller, etc.)
- Add a new `setup.py symlink` target, to use instead of `develop`, which installs scripts and symlinks the library into site-packages, without using setuptools.
- Removes the static script entry points - all our entry points are now automatically generated.