Setting UV_THREADPOOL_SIZE for node v0.10.x

[springmeyer]

If running node v0.10.x we can now bump the threadpool size above 4 via the UV_THREADPOOL_SIZE. It looks like this needs to be set before node is run, but I need to research this more to be sure. We should also keep an eye on joyent/libuv#649 (comment) to see if/when node/libuv move to autoscaling.

[springmeyer]

as noted by @bnoordhuis - setting this on process.env should work, and does (i confirmed by watching the thread count reported in Activity Monitor) if we do it at startup in the index.js, which is now in place.