Make translator comment prefix case-insensitive

Translator comments currently must start with exactly "L10n:". Change
so that's case-insensitive, e.g. "L10N:" or "l10n:" or "l10N:" also
work.

This is for https://bugzilla.mozilla.org/show_bug.cgi?id=868178

(Note: Babel has recently released new versions that break the
tower tests. I pinned the version to 0.9.6 rather than trying to
fix that in this branch.)
Merge pull request #20 from dpoirier/bug-868178-case-insensitive-translator-comments

Make translator comment prefix case-insensitive