Add EventStream.flatMapError
Add Bacon.error convenience function that sends an error and ends

Bacon.once's evil cousin ]:->
Add Bacon.retry
Allow interval by context
Remove everything related to isValidValue from Bacon.retry

Similar effect can (and should) be implemented in the client code,
by transforming the source stream as suggested by @jonifreeman:

Bacon.retry({source: someSource.flatMap(function(x) { return x === "FOO" ? Bacon.error("invalid value") : Bacon.once(x) })})
Merge branch 'retry' of https://github.com/mileskin/bacon.js into mileskin-retry
simplify, include toString
fix Bacon.retry toString
Merge branch 'mileskin-retry'
update relnotes