Backported PROPKA code base to Python2.

Now this same code can be run with either Python2.7 or Python3.

Tests/runtest.py

@@ -4,6 +4,7 @@
 
 from subprocess import call
 import os, re
+import sys
 
 pdbs = ['1FTJ-Chain-A',
         '1HPX',
@@ -14,7 +15,7 @@ for pdb in pdbs:
   print('RUNNING '+pdb)
 
   # Run pka calculation
-  call(['../propka.py','pdb/'+pdb+'.pdb'], stdout = open(pdb+'.out', 'w+'))
+  call([sys.executable, '../propka.py','pdb/'+pdb+'.pdb'], stdout = open(pdb+'.out', 'w+'))
 
   # Test pka predictiona
   result = open('results/'+pdb+'.dat','r')