Integrate the Bower plugin

This adds integration for the [bower plugin](https://github.com/bitovi/system-bower) as well as some tests so that StealJS users can use it easily. The integrations:

1. The path for `bower` is set automatically, unlike in SystemJS, so users can simply do `require("bower.json!bower");` in their configs.
2. If the user provides `bower.json` as the path for their config in the script tag, the bower.json file will be loaded in place of the config.
3. Adds documentation for properties used by system-bower.

This closes #215 and #291

There is a corresponding issue in steal-tools: https://github.com/bitovi/steal-tools/pull/136  This fixes capturing plugins that are used by dependencies of the `@config`. There is **no** special logic in steal-tools to deal with Bower.