Move cache manifest to *.appcache

https://github.com/zhukov/webogram/issues/278#issuecomment-43072649
Improved grouping messages

Now displaying unread icon for grouped messages
Increase group timeout to 5 minutes:
https://github.com/zhukov/webogram/issues/177#issuecomment-42838841
Improved gulp task for cache manifest
Merge branch 'master' into staltzogram