Improved handling of translator comments in Babel plugin

There's no reason an intervening node should clear the translator
comments.  There's already logic that discards the comments if they
occurred too far away from the harvested string.  This way we can write
more natural templates and still have the translator comments that
provide the best translated text.

http://www.makotemplates.org/trac/ticket/225
- changelog for [ticket:225]