Fix steal.dev.log and steal.dev.warn.

- Remove dev.logLevel
- Set default logLevel to 0 in System config
- Get logLevel by passing the key to steal.config, executing steal.config with no params throws.

Closes #220.

@matthewp I noticed that calling 'steal.config()' throws, problem is in https://github.com/bitovi/steal/blob/master/src/config.js#L7, `extend` tries to iterate over `cfg` which is undefined. I'll check if there is an issue already filed, if not I'll file one.