Added a basic repl executable as aws-rb

This repl will default to pry when available, but will fall back to irb when not available.  It accepts a number of runtime configuration options and defaults can be exported to ENV.

By default client requests are logged (in color).  A quick example:

    aws-rb
    AWS> s3.buckets['aws-sdk'].objects['key'].content_length
    [AWS S3 200 0.875184 0 retries] head_object(:bucket_name=>"aws-sdk",:key=>"key")
    => 5242881

You can use all methods of the AWS module directly when using Pry.

You can specify the following ENV variables to control default options:

    AWSRB (pry, irb)
    AWSRB_LOG (on, off)
    AWSRB_COLOR (on, off)
    AWSRB_DEBUG (off, on)

There command line arguments are:

    Usage: aws-sdk-repl [options]
            --repl REPL                  specify the repl environment, pry or irb
        -l, --[no-]log                   log client requets, on by default
        -c, --[no-]color                 colorize request logging, on by default
        -d, --[no-]debug                 log HTTP wire traces, off by default
        -Idirectory                      specify $LOAD_PATH directory (may be used more than once)
        -rlibrary                        require the library
        -v, --verbose                    enable client logging and HTTP wire tracing
        -q, --quiet                      disable client logging and HTTP wire tracing
        -h, --help