Use python3 style relative imports

I changed the imports in all files to import `absolute_import` from `__future__`. This activates the python3 import mechanism. Using this, I was able to get rid of all references to `pycircstat` in the code (would have made renaming the package much easier to begin with and is definitely more elegant).

Signed-off-by: Matthias Kümmerer <matthias@matthias-k.org>