Misc improvements:

Added an optional -S and -T to specify the storage URL and a token to
use for ssbench-master.

ssbench-master will use ST_* env vars as default values for -A, -U, and
-K.

Both ssbench-master and ssbench-worker default to logging.INFO
log-level, but -v/--verbose will set log-level to logging.DEBUG for more
info during a run.

ssbench-master can override the scenario-specified container count
(-c #) and user count (-u #) on the command-line.

Removed ssbench-worker concurrency option and just hard-coded it to 256
(the ssbench-master controls the actual worker concurrency utilized).
Also ssbench-worker will try to set resource.RLIMIT_NOFILE to 1024 (eg.
OS X default is 256 max open files).
Adding a trailing "dev" to version to indicate this is development for a 0.0.6 release in the future.