Feature/python3.2

Resolves issue phihag/ipaddress#15

- Add `from __future__ import unicode_literals` to both `ipaddress.py` and `test_ipaddress.py`.

- Replace all Unicode string literals, i.e. `u'...'` and `u"..."` with plain string literals.

- Replace string literals used in `struct.pack()` and `struct.unpack()` with byte literals for Python 2.6 compatibility.

- Add Python 3.2 support to `.travis.yml` and `setup.py` classifiers.

- All unit tests passing for Python 2.6, 2.7, 3.2 and 3.3; see https://travis-ci.org/tkem/ipaddress.

- Bump `ipaddress.__version__` to `1.0.6`, the version used in `setup.py`; if you release a new version, don't forget to update the version number in *both* files, or better, rewrite `setup.py` to use the version from the module.