Enable Gradle production builds

Really, this enables as many custom source sets as you want, with one
called `production` already defined.
Enable developer-specific build logic

You can now put a dev.gradle file in the root of your project to add any
custom build logic you want. This is useful to, for example, toggle
production builds:

    project.ext.set('env', 'production')
Delete site.gradle

It hasn't been used. Plus, if you really need custom build logic you can
just patch build.gradle, which is what we've been doing anyway.