Support Python 2.7, 3.4, 3.5, 3.6

[mruffalo]

Not many changes were required:

• Relative imports
• Using the six package for a few renamed modules/builtin functions/etc.
• Using the __future__.print_function import in test_concat_gctoo.py

All tests passed under 2.7, 3.4, 3.5, and 3.6, as per an older Travis CI build at https://travis-ci.org/mruffalo/cmapPy/builds/294957365 -- though that build is from an older version of these changes which also included testing Python 3.3. I'm sure everything will work under Python 3.3 also, but the h5py package fails to install under 3.3 on the Travis CI service, so that version probably shouldn't be marked as "supported" without any test coverage.

[levlitichev]

Sorry for accidentally closing! Thanks for the PR, @mruffalo. There are a few things we want to review first, but from a glance, the changes look great.

[oena]

Thanks for taking the time to do this, @mruffalo! Appreciate it.

[mruffalo]

My pleasure! This was much less work than a similar pull request I made in the old l1ktools repository: https://github.com/cmap/l1ktools/pull/14 -- especially since this code was written using modern Python 2.x idioms/style, which really eases the process of making it run under 3.x also. It was also very useful to already have Travis CI testing in place.

[oena]

@mruffalo that was the whole idea behind refactoring it, so I'm very happy to hear it's not just us who find it easier to work with!