Backported PROPKA code base to Python2.

Now this same code can be run with either Python2.7 or Python3.
Added a comment for int-casting of math.ceil().
Improve Python 2 compatability with "future" print_function.

Use python3 version of the print function when running
under python2. Also added "from __future__ import division"
to a few more module files.
use generic she-bang instead of hard-coded python3
Re-packaged with setuptools for standard installation

* 'python setup.py install' will install
  - a package 'propka' (which contains everything that 'Source' contained
    in the original distribution)
  - an executable script 'propka31' (which is identical to the original
    'propka.py' script but it is automatically generated via the setuptools
    mechanism; it uses propka.run.main().
* 'pip install' will also work
* the README.md file was changed to reflect the alterations
* metadata in the setup.py file was added

NOTE: The licence is still unclear!
use pkg_resources to access bundled files

* egg file can remain zipped
* see http://peak.telecommunity.com/DevCenter/PythonEggs#accessing-package-resources
  for details
adjusted test script: all tests pass

* renamed scripts/propka.py to propka31.py so that it can be run
  without import issues
* ran the tests and no difference to the fixtures was detected
added INSTALL and fixed markup in README