first pass at custom notification messages for likes, reshares, RSVPs, etc.

for #18 and #22. plenty left to do; see TODOs in code.

example notification messages:

ĎÚβĨŐÚŚ Dod liked a post linking to http://aaron-gustafson.com/notebook/2014/enabling-webmentions-in-jekyll/ (https://twitter.com/AaronGustafson/status/534866325160878080) (bridgy)
Stephen Jacques liked a post linking to https://snarfed.org/2014-12-04_11815 (https://facebook.com/212038/posts/10101392245246163) (bridgy)
Lauren Buchman RSVPed no to http://indiewebcamp.com/events/2014-01-29-homebrew-website-club (https://facebook.com/200590280142030#625270089) (bridgy)
bear reshared a post linking to https://waterpigs.co.uk/notes/4WfKUc/ (https://twitter.com/bear/status/479721950760419328) (bridgy)
bear retweeted a tweet linking to https://waterpigs.co.uk/notes/4WfKUc/ (https://twitter.com/bear/status/479721950760419328) (bridgy)
Devon LaHar commented on a post linking to https://snarfed.org/2014-11-25_11746 (https://facebook.com/212038/posts/10101380661450183?comment_id=10101386962532763) (bridgy)
[mention] http://bavatuesdays.com/bought-by-some-chinese-concern/ linked to http://indiewebcamp.com/POSSE (webmention)