Fixes #27: Replaced eventlet with gevent.

Eventlet seemed to have trouble with SSL (and there's the wacky stalling
during long-running, large-file upload benchmark runs which this *might*
help), so I replaced eventlet with gevent + geventhttpclient's drop-in
replacement of httplib's `HTTPConnection`/`HTTPSConnection`.

Also made ssbench-worker just use one writer to the beanstalkd result
queue, reducing the max file descriptor requirements for the beanstalkd
process.

Moved ssbench-master's instantiation/connection to beanstalkd into
`Master.__init__()`, and only if a beanstalk host is specified.  This
allows reports to be generated without needing beanstalkd to be
running.

Updated docs and `.requirements.txt` regarding using gevent and not
needing to bump up the max file descriptors for beanstalkd.