Port python bindings to python3

Should be rather easy. However, python2 and 3 adaptors should coexist. With some ```from __future__``` calls they may even share the same code.
```
% python3 setup.py build_ext --inplace 
  File "setup.py", line 11
    print "ERROR: PRECICE_ROOT not defined!"
                                           ^
SyntaxError: Missing parentheses in call to 'print'
```