Initial version of BigMessage.

This is an initial attempt at a big message, basically an SQS message that stores it's payload in S3 and therefore allows for arbitrarily large payloads.

An example:


        import boto.sqs
        from boto.sqs.bigmessage import BigMessage

        sqs = boto.sqs.connect_to_region('us-west-2')
        queue = sqs.get_queue('myqueue')
        fp = open('/path/to/bigmessage/data')
        msg = BigMessage(queue, fp, 's3://mybucket')
        queue.write(msg)

You need to pass in a ``file-like`` object as the ``body`` of the message.  You also have to pass in an ``s3_url`` to specify where the message should be stored.  The ``s3_url`` should be of the form:

    s3://bucket[/key]

If no ``key`` is specified, a ``key`` will be created using UUID's and the body of the message will be stored in the specified bucket using the UUID as a key.  The contents of the ``file-like`` object are uploaded to that key in S3 and a reference to the new object is stored in the message body.  If a ``bucket`` and ``key`` are specified, it is interpreted as specifying an existing object in S3 and no object is uploaded.  The BigMessage will simply refer to the existing object in the body of the message.

When a ``BigMessage`` object is deleted, it will attempt to delete the object in S3.

Feedback on the overall operability of this implementation would be appreciated as well as feedback on the code itself.