Glossaire.sh: update single locale

Proposed fix for issue #4

If called without parameters works as usual (update all locales). If called with one parameter (e.g. 'glossaire.sh fr'), it updates 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