Improve locating of boost-python for Python 3

The name of the boost-python library can vary depending on how it is installed. For example, Homebrew on OS X installs boost-python as  `libboost_python` for python 2 and `libboost_python3` for python 3. I believe Ubuntu installs separate `libboost_python-py27`, `libboost_python-py33`, `libboost_python-py34`, etc.

This commit improves the way RDKit searches for boost-python when installing for Python 3 by using a list of commonly used install names: "python-py34", "python-py33", "python-py32", "python-py31", "python3" are tried in turn, before falling back to the previous behaviour of just using `libboost_python`. A command line option `RDK_BOOST_PYTHON3_NAME` is also added to provide a way to specify a custom boost-python library name.
Simplify locating of boost-python for python 3

Use `${PYTHON_VERSION_MINOR}` to look for the correct boost-python version that corresponds to the specified python interpreter.
Merge pull request #354 from mcs07/boostpython

Improve locating of boost-python for Python 3