Add -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.

```bash
$ aws-rb -q -e "puts AWS.s3.buckets.map(&:name)"
bucket1
bucket2
bucket3
$ 
```

The script above would put the name of each bucket to STDOUT and then exit.  The -q flag (quiet) disables logging.