Introduce HistoryDb.

HistoryDb communicates with a CouchDB instance and wraps a couple of
useful queries for fetching job history.

The history_db_ddocs/views.json file is a reference copy of the
database's design documents.
JobRecorder can handle parsing credentials.
Reorganize dependencies.
Reduce job scheduling delay.
Set queued_at in UTC.
Add pages for history query.

As part of this effort, rewrote the client app in CoffeeScript -- I
find it more pleasant to work in -- and simplified the server app by
making it a Webmachine::Application instance.

Also standardized the names of the response code buckets.
Remove URI normalization.

This is just a mess to track (and seems to do nothing anyway), so I
won't bother with it.