i18n Message improvements

Changes that need to happen:
 - Add a translate() method to Messages to use instead of str() or unicode()
    - translation to system locale done via translate(None)
    - update log.py::ContextAdapter to use translate()

 - str() and unicode() should raise RuntimeErrors to enforce proper usage (translate())
  - and return and error message pointing users to use translate()

 - OpenStack projects should be changed to expect Message objects from _() and handle accordingly

 - catch any Messages and resolve to unicode (using translate()) before they exit any OpenStack code

 - Implement __repr__ for implicit printing cases i.e. print _("hello world") where _() returns non-ascii encodable utf8 bytes

 - support .format() on Messages for now to handle _('Something {0}").format('otherthing') cases
    - translate() needs to support having a Message instance passed as an arg to __mod__ or .format()

- We should NOT support:
  - both __add__ and __radd__ since it implies using translated strings in "improper" ways
  - similarly, __mul__

Related etherpad link: https://etherpad.openstack.org/p/i18nmessages

Some background on the original thinking behind https://review.openstack.org/#/c/46553/

This is a very simple example of what happens if you disable the __str__ method: http://paste.openstack.org/show/48440/

Gerrit topic: https://review.openstack.org/#q,topic:bp/i18n-messages,n,z

Addressed by: https://review.openstack.org/56093
    Translation Message improvements

Retargeting to i-2 because I think there is still some discussion to be held about the implementation. - dhellmann

Addressed by: https://review.openstack.org/59847
    Allow the Message class to have non-English default locales

Addressed by: https://review.openstack.org/62508
    Enable setting up translation handler via CONF

Addressed by: https://review.openstack.org/65385
    Add support for locales missing from babel

Addressed by: https://review.openstack.org/71362
    Return the unicode representation of an object