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.