Optionally using graceful-fs if it's available

[adam-lynch]

Related: #16 and https://github.com/jprichardson/node-fs-extra/pull/95

[mmalecki]

I think we should just add graceful-fs as an explicit dependency and potentially use it instead of controlling concurrency (iirc, concurrency control was introduced to prevent running out of fd's) (cc @AvianFlu)

[jprichardson]

It'd be cool to get this merged in the meantime as this PR doesn't require the addition of any dependency. Any projects that depend up on ncp could then also use graceful-fs and leverage the benefits of graceful-fs in ncp without any practical cost to ncp or its codebase.

[tracker1]

I think this is a nice change... :-) I don't like hard dependencies for stuff like this (considering many don't npm dedupe their repos, and often version settings leave multiple versions, even if they're actually compatible for the use case.