Remove mention of Message objects from public docs

The Message class is not part of the public API of the library, so don't
refer to it in documentation that appears for the public API.

Change-Id: Ie28ce6f8f2c2de351c8a233b7de9dc31abaa9e04
Add a test fixture for translatable strings

Change-Id: I96aec806392672ff69a3af6c3e0a04a621656f78
Document how to add import exceptions

Add documentation for how to add import exceptions so hacking does not
complain about the way the library is used.

Change-Id: Ic33307215abbc38d3d3d2435588f79dc115510fd
Imported Translations from Transifex

Change-Id: I3a8fb8ea66627c929bbb47f16750e7e25e599fa3
Merge "Remove mention of Message objects from public docs"
Merge "Document how to add import exceptions"
Updated from global requirements

Change-Id: I0b2152153a2ea35193140c999dcf061051484861
Imported Translations from Transifex

Change-Id: I806bef9ccffcf914e802b903390c93799aa4d47f
Merge "Imported Translations from Transifex"
Add missing six dependency

Change-Id: I6a661282d6e9381861ff0cf4ba564f76e4812d4a
Closes-bug: #1367761
Remove unused/mutable default args

Having mutable default args is a bad idea
(http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments).

Change-Id: I4a51c1aea04a1d2fd1e03e72a0594d2ee80eb921
Updated from global requirements

Change-Id: If0d58de9daeb46b5190d0c568a590d883aeac9e9
Merge "Remove unused/mutable default args"
Imported Translations from Transifex

Change-Id: If50e6cac440d42f888448959aad337cb341a977c
Imported Translations from Transifex

Change-Id: Id06bb974dba0f29720be3650986bb04e23a2c5e0