Drop support for Python 2.6 and 3.2.

- 2.6 is long out-of-maintenance, and a security quagmire.

- 3.2 cannot be tested on Travis.

Also, point to RTD for docs.

[ci skip]

[ci skip]

Format log only when actually logging.

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

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