Discovered a --py3k option to pylint...

This will warn about constructs in py2 that are not supported
in py3, and should cut down on the # of times I commit something,
push it, and then have to fix it when travis tells me that the
py3 version is broken.
the from __future__ imports change the behavior of print, the
division operator (for ints) and require the use of absolute
imports (which is a convention that we tried to follow anyway,
but which will now be enforced).
disabling warning about tuple unpacking (OK here)
Silence (on module level) warning

logging used % style string formatting, and pylint
warns about passing in args to logging function (to eliminate the
cost of string interpolation when the log message will not be emitted).

We're not worried about this extra cost right now.
See https://github.com/OpenTreeOfLife/peyotl/issues/82
Merge branch 'from-future' into tutorials