Add some important features

Hi!

Please, accept this pull request for adding this features:

- disable-compression - for speed up benchmark (no need spend cpu time to decompress)
- disable-keepalive      - for test scenario where client don't support keep-alive
- cpus                          - useful option flag for avoid using GOMAXPROCS.

Thanks,
Artem Andreenko.
I have a big server. And I want utilize all my cpu's power. It's important if your's server need have 100k req/s. By my opinion, ab and httperf main problem is can using only one cpu core.