Include grunt-digest in dev/prod builds

The digest task will output a JSON file into the public directory
which contains a mapping from filename to filename-cachebusted.

It will also *copy* all the assets in the public directory to their
cachebusted names.
Add cachebusting to spotlight.css

As a proof-of-concept, cachebust this one file. This involves converting
it to use underscore templates rather than mustache. Node.js templating
is painful.
Remove dependency on build:dev from test

npm test (what Travis runs) uses build:development followed by
test:all. These two tasks have some duplication, so just use test:all.

This means that test:all must be responsible for doing some additional things,
like sass:development etc.

Remove nodemon config because we don't use it since #55 was merged.
Merge pull request #113 from alphagov/cachebusting-proof-of-concept

Cachebusting proof of concept