The intersphinx extension can generate automatic links to the
documentation of objects in other projects. For complete details on
intersphinx, see:

http://www.sphinx-doc.org/en/master/ext/intersphinx.html

The sequences are always tuples, not lists.
String attributes are None when missing.

#247

Make initialize() pass extra keyword arguments to LDAPObject

Fixes: #249

For docs, use signatures and doc from the code (and docstrings) using Sphinx
autodoc.

#250

* Python 3 has no basestring, use str instead
* Add a very minimal test to create a SSSRequestControl


Fixes: #255

The documentation build uses docstrings from the code, and so it
needs the code to be importable.
We already use a fake _ldap module for cases where the C compiler
is not available, like on Read the Docs.

Another issue on Read the Docs is that the pyasn1 dependency needs
to be installed. This can't be done automatically via setup.py
install, as that would attempt to build C code.

Instead, add a documentation-only "requirements.txt", which we can
point Read the Docs to.