Added a basic repl executable as aws-sdk-repl.

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-sdk-repl
    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.
Corrected spelling error in comment.
Modified the repl include and require options.

* Now requiring the aws-sdk gem after the $LOAD_PATH
  has been amended.  This allows the user to point
  the repl to a different version of the SDK.

* Now requiring any files specified via -r. This happens
  after the SDK has been loaded and configured.  This
  alows the required library to make use of the SDK.
Changed the usage banner to match the (current) executable name.
Renamed the aws-sdk-repl to aws-rb.

* Renamed the script
* Updated the gem specification executables
* Renamed the ENV prefix from AWS_SDK_REPL to AWSRB
Merge pull request #270 from aws/aws-sdk-repl

Added a basic repl executable as aws-rb