Fixes #153

The items that went in each ``__all__`` are based on what is
documented:

 $ rg --no-filename 'import' docs/ -trst | tr -s "[:blank:]" | sort | uniq | grep zope
 >>> from pprint import pprint
 >>> from zope.interface import *
 >>> from zope.interface import Interface
 >>> from zope.interface import Interface, Attribute, implementer
 >>> from zope.interface import alsoProvides
 >>> from zope.interface import classImplements
 >>> from zope.interface import classImplementsOnly
 >>> from zope.interface import directlyProvidedBy
 >>> from zope.interface import directlyProvides
 >>> from zope.interface import implementedBy
 >>> from zope.interface import implementer
 >>> from zope.interface import implementer_only
 >>> from zope.interface import noLongerProvides
 >>> from zope.interface import providedBy
 >>> from zope.interface import provider
 >>> from zope.interface.adapter import AdapterRegistry
 >>> from zope.interface.declarations import Declaration
 >>> from zope.interface.declarations import InstanceDeclarations
 >>> from zope.interface.declarations import ProvidesClass
 >>> from zope.interface.declarations import named
 >>> from zope.interface.exceptions import BrokenImplementation
 >>> from zope.interface.exceptions import Invalid
 >>> from zope.interface.interface import Specification
 >>> from zope.interface.interface import adapter_hooks
 >>> from zope.interface.verify import verifyObject
 >>> import gc
 >>> import zope.interface

And also some personal judgement about what the public API is that I'm
more than happy to have reviewed.

Fixes #91

Remove many (or most) now-redundant 'pragma: no cover'.

A few minor test changes to get full branch coverage.

* Get as close to 100% coverage as possible.

Without conflicting with #86.

This is almost entirely trivial changes:

- Make dummy methods that should never be called either raise
  NotImplementedError or call self.fail() in tests.
- Standardize the no-coverage pragma
- Add a few pragmas where coverage varies across
  versions/implementations, mostly for the sake of tox (travis should
  hit them all).

* Convert tox to run the coverage command, like travis. A follow up should have it produce coverage for all python versions/implementations.

Fixes #87

* Adjust coverage config for py2 only tests; fix typo; finish replacing self with cls

We exclude interfaces declared with the 'class' statement, because their
methods typically have only a doctstring in the suite, which confuses
the branch coverage machinery.

We *can't* test the classAdvice stuff under Py3k.  Need to deprecate it
before the 4.0 release.