Multi file upload support

[osalabs]

I tried to do minimal changes to support multiple files upload, so this version compatible with old one.
What's changed/added:

• settings.key now is not required. If it's empty - original file name will be used (and so uploaded to the root of S3 bucket, unless settings.key_prefix set - see below)
• settings.key_prefix added. If not empty - prefixed to settings.key to create full S3 "path"
• settings.content_type now empty by default - content type detected from uploaded file mime type
• settings.content_disposition added. By default = true, if false - "Content-Disposition:attachment" header will not be added to S3 file meta.
• settings.on_complete_all callback added. Called when all selected files processed.
• php backend sample updated
• created multi file upload demo page for php backend
• readme updated