Translate schema info

If gschemas are gettextized, their summaries and descriptions can be
translated. Gschema-info tries to find their message catalog and show
translated info.

For now, however, gettext(1) does not seem to support msgctxt. So,
strings with msgctxt like default-values cannot be transted.

https://github.com/jmatsuzawa/gsettings-info/issues/4
README: Add info about l10n and update example
Release 0.2.1