Add Python 3 support

I'm the author of Py6S (https://github.com/robintw/Py6S) which depends on Pysolar. I've recently updated Py6S to be Python 3 compatible, and the only thing that is stopping me releasing the new version of Py6S is that Pysolar isn't Python 3 compatible. So...I thought I'd have a go at making it compatible!

It was actually very easy - the 2to3 tool did the majority of the work, and all that needed changing was a few imports and the print statements. I've checked everything as well as I can, and all of the tests pass on both Python 2.7 and Python 3. I've also updated the trove classifiers list in setup.py so that it will show up on PyPI as compatible with both Python 2 and Python 3.

Would you be happy to merge this?