Glossaire.sh: update single locale

If called without parameters works as usual (update all locales). If
called with one parameters (e.g. 'glossaire.sh fr'), it udates just
that locale.

Also:
* use variables like $release_l10n instead of $local_hg/RELEASE_L10N
and $release_source instead of $local_hg/RELEASE_EN-US when calling
tmxmaker.py
* change from structures like 'if [ "$checkrepo" = true ]' to  'if
$checkrepo' (see
http://stackoverflow.com/questions/3810709/how-to-evaluate-a-boolean-var
iable-in-an-if-block-in-bash)
Merge pull request #81 from flodolo/update_single_locale

Glossaire.sh: update single locale