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

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)
add content snippet to notification messages for posts and replies

...and other tweaks.

example notification messages:

[mention] http://www.kryogenix.org/days/2014/11/30/vouching-for-webmentions-hashing-for-vouches/ linked to http://indiewebcamp.com/Webmention (webmention)
[mention] The Dod liked a post linking to https://snarfed.org/2014-11-24_kyle-mahan-let-them-eat-dogfood-httpgogs-io-2 (https://dubiousdod.org/indie/2014/11/like-of-ryan-barrett-kyle-mahan-let-them-eat-dogfood)
[mention] Tantek Çelik posted 'What apps have #oauth access to your accounts? I created this page to help indiewebcamp.com/FreeMyOAuth Inspired by freemydata.co, expanded ...' linking to http://indiewebcamp.com/FreeMyOAuth (http://tantek.com/2014/324/t1/apps-oauth-access-accounts-freemyoauth)
[mention] Ryan Barrett posted 'One big IndieWeb raison d’être is using your own web site to reply, like, repost, and RSVP to posts and events. You do this by annotating li...' linking to http://indiewebcamp.com/ (https://snarfed.org/indieweb-press-this-bookmarklets-for-wordpress)
[bridgy] Devon LaHar commented 'OK, scary....' on a post linking to https://snarfed.org/2014-11-25_11746 (https://facebook.com/212038/posts/10101380661450183?comment_id=10101386962532763)
Merge pull request #33 from snarfed/pretty-notifs

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