Python 3 compatibility

[DFEvans]

Given that Python 2 will no longer be maintained from 2020, and most astronomical packages are already heavily advocating the use of Python 3, it would be useful if this package could also be made compatible.

In a quick test, the "futurize" tool (http://python-future.org/automatic_conversion.html) seems to do a good job with the package, most of the changes being print, xrange->range, and changes where e.g. keys() returns iterators instead of lists. It should also retain Python 2 compatibility

The only unfixed item I've encountered so far is the definition of AISampler in Sampler.py, due to it inheriting from both object and FileHandler. I believe it's no longer necessary to inherit from object in Python 3, and removing this fixes the issue. The code also still appears to work in Python 2, but I've not done a comprehensive test.