Make OWSLib optional on Python 3

It seems like OWSLib might have a ways to go before it gets Python 3 support, whereas Cartopy is nearly ready _now_. So this PR makes OWSLib fully optional for Python 3 by:
 * Deferring import errors from import time up to the point where classes depending on OWSLib are used.
 * Skip OWSLib tests on Python 3
I was originally thinking of just Travis, but you're right. Since it's documented as optional, it makes sense to skip this test on Python 2 as well, if unavailable.