Benchmark refactor

The three commits have been broken down like so:

1) Update `_bench_timer.js` with additional api features, update existing benchmarks that used it and move them into an api specific directory.
2) Add benchmarks for Buffers
3) Reorganize benchmarks into folders by api category, update easily identifiable ones with the new api, update all that would have been affected by the path change and remove a few that were no longer needed.

I realize how tedious this is going to be, so even though there are a few small things I still need to do (e.g. add Joyent copyright, spell check) figured it would be good to post it now and begin the review process. There are definitely things that need feedback (like where some should go, and why some don't work).

To give an example of some changes in (3), I've updated @isaacs `client_latency.js` to use the new api and now works like the following:
```sh
# --req: number of requests
# --max: maximum number of sockets
# --iter: number of test iterations
$ ../node http/client_latency.js --req 200 --max 50 --iter 10
```