Rewrote with getter and setter support (instead of explicit attributes support). Setter now takes an options object, rather than a single onlyIfEmpty option.
v2.1.0. Improvement.

- v2.1.0 April 10, 2013
	- Removed explicit support for `attributes` and added getter
`get(key)` and setter `set(attrs,opts)` support instead
	- Changed `onlyIfEmpty` argument on `setDeep` into an `opts` object