Should have timestamps for the whole conversation

Currently there's only a time stamp at last bubble of a conversation. It would be good to have more of them. This could be done adding relative times like "an hour ago", "a day ago", "a month ago" to the timeline like ruby's time_ago_in_words.

The time_ago_in_words_function would make a great addition to libhandy's helpers (I'm pretty sure somebody did that alread in the g* universe).