Extracted library code from VisualStock project.
Doc: added README & license.
Cleanup:
 * renamed SharedFormRails module to SharedForm;
 * moved library code to `vendor/assets`;
 * removed unused components.
Upload: moved to `multipart/form-data`.
Upload: refactored a bit.
API: use `data.*.url` instead of `*.url` to display images.
API: use `shared` class instead of `multiple` for `form`s.
Refactoring: options parsing & defaults.
Refactoring: separated into several files.
Doc: updated README to match API changes.
SharedForm: pulled in `master`.