use core-string-decoder as a dependency

@isaacs would you have objections to using *core-string-decoder* instead of core's *string_decoder*? I'll keep it updated in the same way, versioned like Node core (although unlikely to actually change much, or at all between versions).

See discussion https://github.com/joyent/node/pull/6830 about the 0.8 compatibility issues; it'd be awesome if we could solve this in user-land.

@substack already has *string_decoder* in npm but that's not going to help us override the `require('string_decoder')`. I'd like to see if we can unify the two packages (https://github.com/substack/string_decoder/pull/1).