Leave node runtime in tact

This leaves the node runtime in tact so a custom Procfile can be specified that runs a node server. Motivation for this change can be found here: https://github.com/bkeepers/github-notifications/pull/21

Changes:

* No longer move build to `www`
* Install all npm modules before build, then `npm prune --production` to remove development dependencies.

Notes:
* With this change comes the recommendation that the default `package.json` lists lineman in `devDependencies`.
* the `--production` flag to `npm prune` is ignored before `1.3.10`, so the slug will contain all the development dependencies. Setting `{"engines": {"node":"~>1.3.11"}}` in `package.json` will tell heroku to use that version.