prepare for python3 compatibility

Tito is not listed as a blocker for "python3 by default"
at https://fedoraproject.org/wiki/Changes/Python_3_as_Default
but I'm rather fond of tito and wanted to give a shot at
making it python3-compatible.

This changeset does *not* add python3 support since GitPython
is not yet compatible with python3 and may never be according to
https://fedoraproject.org/wiki/User:Churchyard/python3

Nevertheless, here are some contributions...

* Convert all exception clauses to support python 3 and
  maintain compatibility with python 2.5

* Convert print statements to print functions since python3
  does not have print statement

* Add tito.compat lib to compartmentalize incompatibilities
  between python 2.x and 3.x, such as...
  - python3 uses subprocess, not commands
  - python3 uses configparser, not ConfigParser

* Update docs

* Extend travis-ci build matrix
  - add python 3.3
  - remove python 2.5, but only because travis no longer has it
  - add some pep8 checks for python3 compat

* Add editorconfig settings
  If dev optionally installs editorconfig plugin, it maintains
  correct indents, etc, to assure python3 compat

* Add .gitattributes to show better python diff and whitespace err