Eofs

This is a bugfix release also featuring some significant behind the scenes modernisation improvements:

* Lots of important updates from ocefpaf regarding packaging and documenation.
* Bug fix for dask/numpy where from lee1043
* Documentation update from guidov
* Fixes for xarray interface from griverat and rabernat

* Add support for dask arrays [107, 109], thanks to ScottWales.

* Fix for the cdms interface on Python 3 [101, 102]

* Minor updates affecting xarray compatibility, now requiring xarray >= 0.9.
* Modernized testing infrastructure (pytest instead of nose, pycodestyle instead of pep8).
* Python 3 bug fixes.

Improved the detection of time axes in the iris interface.

Release notes:
- Added an [xarray](http://xarray.pydata.org) interface allowing use of `xarray.DataArray` objects with eofs.
- Fix bug in setting `ddof` in `eofs.tools.iris.covariance_map()`, previously it was always `1` regardless of the `ddof` keyword.
- Better Python 2 and 3 support without relying on 2to3.