[WIP] add jira (and other trackers) support for slurper

Changes:
- refactor slurper to support handlers based on the provided configuration
- a story is just a simple value object - move away from `Activeresource`
- a handler will know how to persist the story in it's configured backend

Some benefits of this refactor:
- adding a new persist handler is now trivial (implement the `supports` / `handle` protocol)
- default values can now be provided for other fields besides `requested_by`
- fully backward compatible with the old slurper