In WeakSet, use the `valuerefs` method, added in Python 2.5 instead of accessing the internal `data` implementation detail.

This lets us support Jython.

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

This was seen on winbot[*]:

  ValueError: path 'docs/_build/' cannot end with '/'

with a traceback ending in "c:\Python26_32\lib\distutils\util.py", line
201, in convert_path.

[*] http://winbot.zope.org/builders/transaction_py_265_32/builds/845/steps/buildout/logs/stdio

I cannot reproduce the error locally, using Python 2.6.9.

Also update MANIFEST.in to exclude files that shouldn't be in the sdist.

Also, work around edge case on Travis where files aren't found.

Normalize interface imports, and exclude them from branch coverage,
because the coverage shows false negatives for method docstrings of
interfaces.

Downloaded from: https://bootstrap.pypa.io/bootstrap-buildout.py.

Fixes #4.

README: Correct docs URL

One way to reproduce these in the wild was to run the ZODB test suite on
Python 3.3 in the py3 branch.  Example failure:

Error in test checkPackWhileWriting (ZODB.tests.testMVCCMappingStorage.MVCCMappingStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python3.3/unittest/case.py", line 385, in _executeTestPart
    function()
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 281, in checkPackWhileWriting
    self._PackWhileWriting(pack_now=False)
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 208, in _PackWhileWriting
    transaction.commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_manager.py", line 111, in commit
    return self.get().commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_transaction.py", line 286, in commit
    self._synchronizers.map(lambda s: s.afterCompletion(self))
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/weakset.py", line 55, in map
    for wr in self.as_weakref_list():
RuntimeError: dictionary changed size during iteration

Thank you, https://gist.github.com/mgedmin/4277075

- Denominate files to indicate ReST.

- Drop .bzrignore.

This requirement is needed to ensure a total ordering, e.g. in Py3k where
strings and ints are not comparable.

Suppress warning under 'tox -e docs'