Add an index for sprint related cards

In order to get the cards for a sprint I needed to add a custom index to
the PouchDB so I defined the view function in the sprint model file and
added code to Database.js to load it and create a design doc in the
database. This keeps any app specific code out of Database.js.
Add actions to model/database

To implement the action of assigning a card to a sprint I made a pair of
functions that update the the value associated with the 'sprint' leu in
a card document. I added these functions to the sprint model and updated
Database.js to load and partially apply them so they have access to the
PouchDB instance created at runtime. This keeps app specific logic out
of Database.js.
Add design documentation to the sprint model

Sketching some thoughts about how the sprint model will retain
information about how cards are moved in and out of the sprint.
Ignore npm debug log

Generated by npm actions, like installation of packages, and should not
be committed to the repo.