Internationalisation of emails

(Ready for feedback but not for merge)

Here's a simple attempt at internationalization/localization of emails, as discussed in #308.  Highlights:

  * caller can specify preferred language in "lang" POST parameter or the "accept-language" header
  * email templates have access to "gettext" helper which can suck in the relevant strings
  * uses our own i18n-abide module under the hood (although it's not an ideal fit API wise)
  * extensively localized for my local dialect ;-)

I'll do another pass on this next week and clean it up some more, but wanted to get it out there for initial feedback.  @dannycoates @ckarlof f?

@ozten I did a little wrapper around i18n-abide to make the API more suitable for our purposes here.  If you get a chance, please take a look and let me know if something like this could be pushed into the upstream API.