Python 3 support

Updates for compatibility with Python 3, while raising the minimum required Python version to 2.6.

Note that the big diff for the test_basic.py module was due to line-ending fixups (which was separated out from the Python 3 changes, see individual commits).

I added a couple additional tests, and (using tox) all tests are passing for Python 2.6, 2.7, 3.2, 3.3, and 3.4:
```
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  congratulations :)
```

Change could use some more eyes and testing, as test coverage with existing unit tests is below 80%:
```
Name                    Stmts   Miss  Cover
-------------------------------------------
xlsx/__init__             173     34    80%
xlsx/formatting            51      4    92%
xlsx/tests/__init__         0      0   100%
xlsx/tests/test_basic      77     11    86%
xlsx/timemachine           10      2    80%
xlsx/xldate                78     42    46%
-------------------------------------------
TOTAL                     389     93    76%
```