Simplify the testing workflow

This ditches Grunt for simple `npm run` calls and `&&` short-circuiting.
Fix the logic for session persistence

I should be parsing the configuration JSON file after reading it from
disk.
Stub the Reddit API calls in the tests

This uses `mocha-make-stub` to stub the HTTP requests made to the reddit
API when testing the `reddit` module. Since the reddit API is rate
limited, this is pretty much the only way to go.
Cache the username after logging-in and reuse it

This stores the `user` credential after logging-in in the `reddit`
object, so that `reddit.saved` can have its `username` parameter default
to the logged-in user.
Fix problems in the `cli` module

This adds the missing dependency `optimist` to the package.json
manifest, makes the `login` step run on all actions except help and
no-ops and adds a simple `saved` action for viewing saved posts.
Refactor the `sessions` module

This removes the `jsonfile` module from the project and updates tests
accordingly. It also fixes `ENODENT` errors' treatment.
Update the `help` message

This isn't definitive, but it's nice to keep track of where we are.