doc/api.rst: remove reference to `miniswift` mod

This module has been removed, and the doc build was displaying a warning
about the missing module.
zpm: Add docstring to ZeroCloudConnection class
doc/conf: Allow hyperlinking to python stdlib

This is useful for references stdlib modules like `tarfile`.
tox.ini: add sphinx-build flags for strictness

The following flags are now specified when docs are built:
    -n: nitpicky mode
    -W: turn warnings into errors

See http://sphinx-doc.org/invocation.html for more info.
Merge pull request #152 from larsbutler/stricter-doc-builds

Stricter doc builds