install owslib only on python 2 for now (this will intentionally leads to errors on python 3)
Make OWSLib optional.

The ImportError is deferred until the classes are actually used.
Skip OWSLib tests if unavailable.
Merge pull request #495 from QuLogic/owslib-optional

Make OWSLib optional on Python 3
Make floats in proj4 strings fixed precision.

Previously, the default precision was used, but Python 2 and 3 differ on
what that is. This causes small variations in the results.

This change is expected to fail since tests are not updated.
Update proj4 test strings with increased precision.
Update Robinson tests for increased precision.
Update Transverse Mercator tests for new precision.
Update Lambert Conformal tests for new precision.
Update linear ring test for new precision.
Update plots for new precision.