Also rename decorator to _decorator per discussion.

For two reasons:

1) DRY
2) Ability to cross-reference :mod:`zope.event.classhandler`.

Reference zope.component docs on configuring events

Fixes #6

I'm working backwards from zopefoundation/zope.lifecycleevent#7
building up a chain of references :)

Also reorganize the main "usage" documentation for more narrative
readability. Make it the landing target for zope.event module
references.

- Add Sphinx documentation.

- Add Sphinx builder recipe.

- Keep the (empty) docs/_build in the checkout.

- Move narrative docs to 'docs', remove from unit tests.

  The snippets in the docs get tested as part of building the HTML docs
  via the 'make-docs' script.

- Move full docs out of the --long-description.

  We want to be able to make full use of Sphinx markup, which confuses
  the PyPI interface.  Instead, the README.txt now points to the docs.

- Let 2to3 know about the new location of the docs.

- Add docs for hacking on the package.

  This page is actully pretty generic.  Assuming that we make it true for
  any ZTK package, it might be better off in the top-level ZTK docs.