Parser: allow multiple comments

Since it’s only used for reference locale, read metadata in a second pass on the file content, consider comments as an array.

I assumed there could be only one comment, realized it's not the case this morning with the [current version of home.lang](http://viewvc.svn.mozilla.org/vc/projects/mozilla.com/trunk/locales/en-GB/mozorg/home.lang?revision=130241&view=markup). This has two consequences: we read only one comment, we don't [associate the string with the tag](http://l10n.mozilla-community.org/~pascalc/langchecker/?website=0&file=mozorg/home.lang&action=translate) because the TAG binding is too far.