PYTHON-24: Group operations (add and remove)
docs: fix sphinx warnings and expand on docs for get_devices()

This started as just addressing warnings, but I decided to add some
examples and further clarification to get_devices() while I was making
changes.  This is likely to be one of the methods that almost every
developer will use and I thought the addition of some additional
examples was justifiable.
docs: add note about pyenv dependencies for running ./toxtest.sh
Invalidate device cache after adding and removing from groups.
Merge pull request #19 in PYTHON/python-devicecloud from PYTHONDC-24-group-operations to master

* commit '2e83ec30bc733151dcf6dc90e4021a95bfd00ef8':
  Invalidate device cache after adding and removing from groups.
  PYTHON-24: Group operations (add and remove)
Merge pull request #20 in PYTHON/python-devicecloud from minor-doc-updates to master

* commit 'b52b0255b45f119fa173656a5d61cc8b9fb9e909':
  docs: add note about pyenv dependencies for running ./toxtest.sh
  docs: fix sphinx warnings and expand on docs for get_devices()