When resolving requirements, we now pass the list of extras we're using along
to Marker.evaluate, since we want to include the extra's requirements in our
list of required packages. This is sort of papering over the underlying issue;
namely, that the dependency map for dist-info distributions looks like:

  { None      : ['common_dep'],
    'my_extra': ['extra_dep; extra = "my_extra"'] }

If we eliminated 'extra = "my_extra"' when creating this map, the problem
would go away because the WorkingSet would no longer treat `extra_dep` as a
purely optional dependency. However, this would require copying and
manipulating Requirement objects, which is somewhat more complicated than the
current solution.

Use py.test to launch pytest, rather than python -m test, preventing the empty path from being added to sys.path per pytest-dev/pytest#2104. Fixes #852. Also use 'usedevelop' as the setuptools.tests.fixtures aren't available in a standard install.

docs: reorder ToC; update stray Bitbucket links

Metadata and options are now could be set in configuration files

Spelling fixes

Tell unittest.main not to exit, fixes #850.