Added Amazon S3 file uploader support.

This patch adds Amazon S3 support via its HTTP POST API: http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingHTTPPOST.html

For testing, I can provide access keys to a test bucket (though I'd rather not post them publicly - shoot an email to alan@alanedwardes.com).

Any questions, comments, suggestions, please let me know. I've tried to be as in-keeping as possible with existing code/UI, but there might be areas that need changing.

![sharex-s3](https://f.cloud.github.com/assets/110954/2527784/9b074174-b508-11e3-8900-3af2bb57b57a.png)

Cheers.