Replaces .next() method calls with function calls.

This improves (provides, perhaps?) Python 3 compatibility (at least for
3.3+) with prettyplotlib. Python 3 promoted next() to a full function of
the language, and consequently removed the .next() method from
iterables. next() was backported to Python 2.6. See
http://docs.python.org/3/whatsnew/2.6.html#other-language-changes.
Matplotlib has required a minimum Python version of 2.6 since v1.2,
thus, prettyplotlib loses no compatibility.
Python 3 compatibility for uppercase/lowercase characters.

This introduces a dependency on the six library, but this is already a
dependency in the chain of matplotlib. I have not added this dependency
to the install_requires list of setup.py because presently it is only
needed for the test code, not for the prettyplotlib library, itself.
Adds classifiers to the package.

For available classifiers, see
https://pypi.python.org/pypi?%3Aaction=list_classifiers
Merge branch 'classifiers' into python3-compatibility
Updates setup.py to indicate Python 3 compatibility.
Merge pull request #12 from gotgenes/python3-compatibility

Make prettyplotlib Python 3 compatible