Setup tox for Python 2.6 and 2.7

These are the 2.x versions that are continuously integrated using Travis CI,
thus it makes sense to test them with tox as well.
Convert most doctests into unit tests

Fixes issue #14. This also gets rid of some tests that suffer from
indeterministic dictionary outputs.
Make string formatting compatible with Python 2.6

In Python 2.6 the new format string specification did not allow empty
positional arguments, e.g. "Vector{}". Thus this needed to be specified
explicitly.
Add numpy as a dependency explicitly
Add Pythons 3.2 and 3.3 to tox config
Fix some 2.x vs 3.x issues

ConfigParser -> configparser, imap -> map
Merge branch 'fix/tests_2.x' into fix/python_3.x

This merge is necessary for regression testing. Since introducing Python 3.x
compatibility could potentially break functionality on Python 2.x it is
important to integrate this branch, which fixes tests on 2.x, into the 3.x
branch.
Be compatible with Python 3.x

A couple of changes had to be made, especially concerning iterators vs. lists
and bytes vs. strings, two of the major changes in Python 3.x. Some issues
could be resolved using six as compatibilty library. Therefore, this was
introduced as a dependency.
Merge pull request #18 from aepsil0n/fix/python_3.x

Fix tests for Python 3.x