Specify extras for gettext message extractors

Added "babel" and "lingua" dependency entries to the setuptools entrypoints
for the babel and lingua extensions, so that pkg_resources can check that
these extra dependencies are available, raising an informative
exception if not.  Pull request courtesy sinoroc.

Fixes: #304

Closes: #305
Pull-request: https://github.com/sqlalchemy/mako/pull/305
Pull-request-sha: 9a1c9a2242a41b553af2524381ab69c2e404a778

Change-Id: I973081a17ed27bf2f954d73133b0d52855bcacff