add po merge script
update locales with the latest strings
new po merge grunttask and README for locales
Merge pull request #579 from mozilla/issue-576-merge-po-files

merge po files

@zaach, this looks really good. The string extraction works well, manual testing added new string to the .po files in each subdirectory, and the directions are clear.

My last concern is the diff size every time strings are extracted because of changing line numbers.

r+