Docs: fix a docstring to match the actual default
Rename {,__}nbands to {,__}numbands
Name input and output signal args more sensibly
Docs: convert docstrings to the numpydoc syntax
Docs: fix incorrect capitalisation
Docs: correct the description of the w_co argument
Docs: remove optional from the description of w_co

The argument w_co is only optional in __init__, and the "optional" bit should
stay out of the description, anyway.
Doc: format the formulae better
Doc: document the return value of LTISys.filter()
Docs: move ref to sp.signal.lfilter to "See Also"
Docs: shorten the module summary
Add a setup.py and sphinx configuration

Add a setup.py that also builds the documentation with sphinx.  Also add the
corresponding sphinx configuration.
Docs: use "autosummary" on docs/modules/
Docs: refer to the filter constraints in "Notes"
Docs: document constructors in the class docstring

Following the numpy docstring conventions, document the arguments to
constructors in the class docstring.