Asynchronous NIF Rewrite

Using [gburd/lmdb](https://github.com/gburd/lmdb) and [basho-labs/wterl](https://github.com/basho-labs/wterl) as a reference, I rewrote this NIF to use [async_nif.h](https://gist.github.com/gburd/4121795) so that it is *less* blocking than the original synchronous version.

It should be a drop-in replacement to the 0.2.x version.  I added an additional function, `ekstat:close/1`, for forcing [kstat_close(3kstat)](https://www.illumos.org/man/3KSTAT/kstat_close) to be called.

All of the tests are passing for me on SmartOS joyent_20140221T042147Z with Erlang R16B02.

I'm hoping to do something similar to [project-fifo/erltrace](https://github.com/project-fifo/erltrace) when I have the time.

Hopefully you find this useful!