Fix errors with isnull() that appear with pandas 0.21

[shoyer]

xref #1542

• Tests added / passed
• Passes git diff upstream/master | flake8 --diff
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[shoyer]

The culprit is the truly monstrous _func_slash_method_wrapper function where we read names from methods if need be:
https://github.com/pydata/xarray/blob/master/xarray/core/ops.py#L211-L216

We should have a more sane way to add methods on xarray objects, but this fix will have to do for now.