Added a -e option to aws-rb.

You can now execute commands using aws-rb.  Passing commands
with the -e option will prevent the REPL from launching and will
exit instead.  You can pass multiple -e options.

    aws-rb -q -e "puts AWS.s3.buckets.map(&:name)"

The script above would put the name of each bucket to STDOUT and
then exit.  The -q flag (quiet) disables logging.
Passing the -e flag to aws-rb now defaults quiet mode.
Now joining execute options for aws-rb
Merge pull request #299 from aws/aws-rb-execute

Add -e option to aws-rb