Add output for the number of errors and retries

This patch enables ssbench to show # of errors and retries
for availability calculation.
This patch enables to show # of errors and retries and
retry rate in addition to current output.

Each item of additional output means as follows:

Errors: The sum of errors which occur during benchmarking.
Retries: The sum of retries for each job.
         (Count up by worker when catching ignoring http
          errors or socket errors)
Retry Rate: The proportion of retries. (# of retries / # of requests)
            i.e. The rate may be higher than 100 %.

Additionally, semantics of 'Count' is changed
to include # of errors.
(Previously, ssbench excluded errors from 'Count')

[Output Example]
For a run which attempted 10 READ operations, with one Exception
with 10 retries, 9 successes with 4 retries.

'''
  READ
         Count:    10 (    1 error;     14 retries:  140.00%)  Average requests per second:  88.9
'''
Minor tweaks to PR #74.
Some flake8 fixes.
Allow multiple -S <storage_url> args.

When more than one -S <storage_url> is specified, one is chosen at
random for each connection to override any storage URL returned from
authentication.  fixes #40

Drive-by: fixes #48
Updated docs; bumped version to 0.2.17
Bind to 127.0.0.1 if spawning local workers.

If a bind IP isn't specified, ssbench-master will now bind to 127.0.0.1
when spawning local workers.  This will keep any existing remote workers
which are still "pointing at" the ssbench-master host from getting work
jobs when spawning local workers.

Bumping version to 0.2.18.
Calculate some requests-per-worker-id stats (printed at DEBUG level)
Improve multiple -S support.

Pick a storage URL for each request (better multiple -S support).

Lowered default ssbench-worker concurrency level to 64.

Bumped version to 0.2.20
Add cleanup-containers command to ssbench-master.

Added cleanup-containers command to ssbench-master to delete all
ssbench-created containers and objects.  fixes #66.
Only create initial files if not already present.

fixes #65
Bump version to 0.2.21
Allow ssbench-master to override worker block_size

Allow ssbench-master to override the default work block size used for
GET/PUT operations (-b BYTES or --block-size BYTES)

Always print the distribution of requests per worker-ID in the report.

Document the container_base key in the scenario file and allow a
non-default value for that to be passed into the cleanup-containers
sub-command.

Bump version to 0.2.22.
Simple typ-o fix to make -T <token> work

Signed-off-by: Peter Portante <peter.portante@redhat.com>
Add installation section for RHEL 6.4

Signed-off-by: Peter Portante <peter.portante@redhat.com>
Merge pull request #77 from portante/fixes

Fix -T token use and add RHEL 6.4 installation section
Update OS X install instructions.