Add a package.json for managing frontend dependencies

This allows us to install all but compass locally in the project rather
than relying on global executables.
Install frontend dependencies in bootstrap
Update webassets paths in configuration files

These now use project local dependencies rather than relying on them
being installed on the users PATH.
Update INSTALL docs to instruct on node installation

This reduces the number of installation steps for most platforms. Node
installation instructions taken from the Joyent wiki[1].

[1]: https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager
Merge pull request #1271 from hypothesis/node-modules

Manage frontend dependencies with npm