chore(client/js/uploader.s3.js): #897 - remove console.log
feat(upload handlers): #897 - allow modular support of uploads to S3
- stubs for XHR and Form S3 upload handlers
- refactor base upload handler to allow uploader modules to specify specific upload handlers to use based on the UA
- refactor upload handler creator to allow upload modules to pass specific additional options to the specific handler
- reference new upload handlers js files in build