Attempt to fix buildxpi.py

See
https://groups.google.com/forum/?fromgroups=#!topic/mozilla.dev.l10n/sZrgTqBX6I4
Remove incorrect parameter
Don't test Trados if we don't have BeautifulSoup
Actually run unit test for optrecurse

py.test will not "collect" a class for testing if it has __init__ method,
even if it otherwise matches the necessary patterns:
http://pytest.org/latest/goodpractises.html#conventions-for-python-test-discovery
Prevent import exceptions in Sphinx autodocs

add Mock()ed PyLucene and provide its VERSION
don't try to access fixtag method of ElementTree if it is Mock()ed
don't raise ImportError for cPO/fPO if no gettext PO library found
Don't confuse pytest with build/ copies of tests

also skip docs and .git to speed tree-walk (nothing there to import anyhow)