Update install instructions

[zakv]

Right now the docs suggest running python setup.py develop to perform an editable install, but the better approach is to use pip install -e <path_to_mloop>. See e.g. https://stackoverflow.com/questions/30306099/pip-install-editable-vs-python-setup-py-develop.

Similarly, the suggested testing command at the moment is python setup.py test which is now a deprecated approach (see pypa/setuptools#1684, pytest-dev/pytest#5534, pytest-dev/pytest#5546). The command pytest runs the tests, so that should be the suggested method.

[zakv]

This issue came up in #111.