Fix package list inconsistency caused by namespace package on Python 3.5

Unload all pkg_resources modules and not just the main module.

Fix EBNF for package specifications.

fix an issue for bdist_wininst with gui_scripts:

update certifi to 2015.11.20

The script header was always generated with non-gui executable.
This was caused by by adjust_header assuming the executable is always an
absolute path.

Fixed by using find_executables() from distutils.

--HG--
branch : bdist_wininst_gui_scripts

--HG--
branch : sync_certifi

namespace package will be skipped during installation. Since Python 3.5, .pyo files are removed and new .opt-1.pyc (and .opt-2.pyc) files are introduced [1]. However setuptools does not understand that new naming therefore the corresponding foo.opt-1.pyc is still added into package list (via --record). The inconsistency leads to a packaging error.

[1] https://www.python.org/dev/peps/pep-0488/

Update tox tests so they run the full suite of tests using the canonical technique. Include .tox in norecursedirs to avoid testing all the dependencies and environment. Update docs to reflect findings (https://bitbucket.org/spookylukey/setuptools/commits/88339d2e4af661a60a42bc14c4ff3e68cd19fa21#comment-2558164).

Draft a test for testing the new expectation for develop command (and general functionality when console scripts are present).