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.
I can change it, but a large part of the ExcelMagic module is defining a list of symbol names, so the resulting import statement would be really quite long. This is one case where I think importing * makes sense.