S3 based file uploading

- [ ] Ready for merge

Allow pro users to drag and drop assets inline to JS Bin and uploads to S3.

- [x] Direct upload to s3
- [ ] CodeMirror insert feedback (ala github)
- [ ] Textarea insert feedback (ala github)
- [ ] JS, HTML and CSS support
- [ ] Preprocessors (likely to be dropped)
- [ ] Account panel to "manage" assets (at least view)