first implementation of localization using gettext

This implements gettext localization at the most basic level. Not all strings are translatable yet, and plurals are not handled. This also does not auto-generate or install the localization files (e.g. in setup.py). But this should be merged now since it touches so much of the code to add the _() function. So maintaining this in a MR will mean lots of painful rebasing.

This also explicitly sets the minimum Python version to 3.4 and includes some other minor cleanups.