Blog post introducing persisted map/reduce

I'd like to get feedback on this, but I think it might be a nice gentle introduction to the new map/reduce API in 2.2.0.  ~~[Here's a static version on S3](https://nolanlawson.s3.amazonaws.com/pouchdb/www/20140423/nolan-blogpost-4/index.html)~~ (updated below) so you don't have to build the site yourself.

And this should help us make a decision on #2017.  For this blog post I'm assuming `putView()`, but I have to admit, it does look a little gnarly in places (still not as gnarly as pure ddocs, though).