Use `python -m pydoc` as default for `g:ref_pydoc_cmd`

`pydoc` is not being installed into a virtualenv
(see https://github.com/pypa/virtualenv/issues/149), but `python -m
pydoc` can work around this.
Merge pull request #49 from blueyed/pydoc-use-python-m-pydoc

Use `python -m pydoc` as default for `g:ref_pydoc_cmd`