Reduce dependencies and allow configuration

This removes Bower and the [`closurecompiler` package](https://npmjs.org/package/closurecompiler) as dependencies.

Bower was used to download the Closure Library.  This is now handled with the smaller [`get-down` package](https://npmjs.org/package/get-down).

The `closurecompiler` package did more than necessary (replaced with `get-down`) and wasn't configurable.

Versions of the Closure Compiler and Library can now be specified in `closure-util.json` configuration files or environment variables (see the readme for detail).