modernized installation procedure

- set VERSION to 3.1.0 (introduce a patch level 0 so that it becomes
  easier to publish packages with small fixes, eg for installation)
- got rid of ancient ez_install: just use pip
- updated README and INSTALL
- updated classifiers in setup.py (license: LGPL v2.1 and other meta data);
  updated URL to point to http://propka.org because unlike http://propka.ki.ku.dk
  it is not completely dead and has a link to the pdb2pqr propka server as well
  as the GitHub page

README.md

@@ -16,17 +16,6 @@ in publications:
 See [propka.ki.ku.dk](http://propka.ki.ku.dk/) for the PROPKA web server,
 using the [tutorial](http://propka.ki.ku.dk/~luca/wiki/index.php/PROPKA_3.1_Tutorial).
 
-## Modifications 
-
-This release of PROPKA 3.1 was modified by Oliver Beckstein
-<oliver.beckstein@asu.edu> from the released version.
-
-* Included patches from
-  https://github.com/schrodinger/propka-3.1/tree/python27-compat to
-  make it compatible with Python 2.7
-
-* Packaged for installation with setuptools.
-
 
 ## Installation