Enhance development experience disabling the background script

[fabiodrg]

I am not sure about other browsers, but on Firefox, every time I change JavaScript code I have to reload the extension manually. That triggers the onInstalled event, thus it shows the changelog on a new tab. My suggestion is to take advantage of the existing development environment powered by Gulp and the environment variables. Something like, if environment is development, then change the manifest.js and remove the background script entry. With the plugin gulp-merge-json, already installed, seems straightforward.