Python 3 support

Following discussion on PR #32, this adds Python 3 support without any external dependencies. Python 2.6 and above and Python 3.3 and above should work; older versions will not.

The tests are passing on Python 2 and 3; I haven't tried the examples.
It probably wasn't. `_REC_ID` is not defined anywhere outside a class ([code search](https://github.com/python-excel/xlwt/search?utf8=%E2%9C%93&q=_REC_ID)). I don't think anything called this method, so you presumably didn't notice.