Add proper python README

- Convert README to restructuredtext format so it displays correctly on PyPI
- Update email address to current mailing list
- Tidy up setup.py
Ensure build_ext runs before build_py

The setup.py build and install commands run a number of subcommands, including build_ext, which runs SWIG to generate openbabel.py, and build_py, which copies openbabel.py and other python sources to the build directory.

The default ordering is for build_py to be run before build_ext, causing errors as build_py attempts to copy openbabel.py before it exists. This change ensures build_ext is always run first at the start of both the build and install commands.
Make pkg-config check more robust and allow manual override

Add a sensible fallback if pkg-config fails to locate the Open Babel include and library directories, and a helpful error message if that fallback also fails.

Allow Open Babel include and library directories to be manually specified as command line options to setup.py using the build_ext command with -I and -L options.
Change python bindings package name to openbabel-python
Add more info to python README and revert package name change

Change the python bindings package name back to 'openbabel', but make clear in the Python README that this is just the python bindings, not the main Open Babel library, just in case anyone stumbles across the PyPI page by accident.
Bump python bindings version to 1.8.1
Merge pull request #65 from mcs07/pythonsetup

Improvements to Python setup.py