Add zope.interface as requirement.

Fix this travis error, specifically:

  File "/usr/local/lib/python2.7/dist-packages/zope/component/__init__.py", line
  19, in <module>
    from zope.interface import named

  exceptions.ImportError: cannot import name named
  globaleaks.tests.test_transaction
GlobaLeaks manpages: employ globaleaks.manpages file.

Remove rude copy of the manual page, and use the standard <package>.manpages
file to install the manual page.
use [ ] instead of [[ ]].

Why new test when the old one works ok and is used all over the rest of the
file?
Fix #958.

Provide a /etc/default/globaleaks.d mechanism, and do not hardfail if a default
configuration is not found.