Issue 380

PR related to issue https://github.com/nzakas/eslint/issues/380.

- Moved the globals of the enviroments, contained in `environments.json`, to a `globals` section.
- There is an optional `rules` section per environment (see, for instance, node).
- When a config is loaded, the rules are merged with the following override order: base, env and user.
- Added tests.