Also update narrative docs for Choice and vocabularies. Fixes #14.

Fixes #18

Some other vocabulary fixes while I was in there:

- Make TreeVocubalary.fromDict only actually assign titles when a
  title is given. This makes it consistent with
  SimpleVocubalary.fromItems().

- Give equality and hash methods to SimpleTerm and SimpleVocubalary.
  This was a confusing stumbling block in the implementation of #51.

Fix Iterable and Container to conform to their interfaces.

Previously they would fail like this:

  File /home/travis/build/zopefoundation/zope.schema/src/zope/schema/_bootstrapfields.py, line 522, in fromUnicode
    raise self._validation_error(*last_exc.args).with_field_and_value(self, value)
TypeError: __init__() missing 1 required positional argument: 'literal'

In descending order of generality these are ``Number``, ``Complex``,
``Real``, ``Rational`` and ``Integral``. The ``Int`` class extends
``Integral``, the ``Float`` class extends ``Real``, and the
``Decimal`` class extends ``Number``.

Generalize the parsing of numbers in the Number class.

Add tests and document them.

Run the field and bootstrap field doctests as part of unittesting, not
just under tox with sphinx, and make them run on both Python 2 and 3.

Fixes #49