install gettext on OS X for msgfmt

as already mentioned by this doc, gettext provides msgfmt, which is
required for i18n validation via tox.

Change-Id: I4cae5fddc42897615554746f946ed4f490dc1ca5
Add instructions for removing pyc files to docs

So an issue I hit when following the getting started docs for developers
is that I forgot to remove old pyc files and that can cause
keystone-manage db_sync to throw errors.  I added some extra doc that
explains how to remove these files should this situation be encountered

Change-Id: Ib41c62e99faa7c82f89ab3c5bfa0d740f426829a
Merge "Add instructions for removing pyc files to docs"
Merge "install gettext on OS X for msgfmt"