Capture build dependencies in package.json

This allows others to install the dependencies for compiling Marathon's
JavaScript. The build must be executed from the root assets directory,
src/main/resources/assets.

    # Work from the root of the assets
    cd src/main/resources/assets

    # Install dependencies
    npm install

    # Build JS
    ./bin/build

Fixes issue #121
Merge pull request #122 from mesosphere/npm-modules

Capture build dependencies in package.json