api: configure datasources via loopback-boot

Move the configuration of datasources to datasources.json,
add environment-specific configuration files.

Modify api/app.ap.js to setup datasources via loopback-boot.
html5: enable sourcemaps in debug env
html5: load datasources from datasources.json

Modify configure.js script to merge all datasource config files
and create a single browserify-enabled file.

Modify html5/app.html5.js to dynamically register datasources
provided in the config file.

Modify configuration of the `watch` task to include json files in
the change detection list.