Add support for Sublime Text 3

These are the changes I had to make in order to get this plugin to work with ST3. They are pretty straight forward so I don't think I broke anything, but I've only tested the import sorter and I haven't tested backwards compatibility for ST2

I removed ordereddict.OrderedDict because it doesn't seem necessary to support for python < 2.4, and UserDict was removed in python 3.

I hope this is useful to people.