Fix up Python2 regressions in PR 417

[jvansanten]

#417 introduced some regressions with Python2. To whit:

• add_property() attempts to add docstrings to functions passed to it. test.properties binds an existing instance method again as a getter, which works fine on Python3, where klass.member returns the underlying function object. On Python2, however, this returned an unbound instancemethod with an immutable __doc__ attribute. Work around this by skipping docstring generation on PyMethod objects.
• test.nested exercises the __qualname__ attribute, which didn't exist before Python 3.3. Use doctest option flags to make examples conditional on the Python version.

[stefanseefeld]

Thanks @jvansanten , the develop branch is stable again. I intend to merge develop to master shortly, so all of that work should be part of the next boost release. Thanks again for your contribution !