Improvements to Python setup.py

These commits provide some improvements to the python bindings.

- Converted the python README to [rst](http://docutils.sourceforge.net/rst.html) so it displays nicely formatted on [PyPI](https://pypi.python.org/pypi/openbabel).
- Changed the order of subcommands so `build_ext` comes before `build_py` when running the `build` or `install` commands. This ensures SWIG is run to generate `openbabel.py` before `build_py` attempts to copy it into the build directory.
- Added some error checking and fallback for if `pkg-config` fails.
- Allows the Open Babel library and include directories to be specified manually using command line options, i.e. `python setup.py build_ext -I/usr/local/include/openbabel-2.0 -L/usr/local/lib`.
- Warn if the Open Babel version is less than 2.3.0, as it isn't supported.
- Added better error message if SWIG fails to find Open Babel.