Tweaking remote-form and remote-method to become tg-remote

- Rename `remote-method` and `remote-form` to be simply `tg-remote` (namespaced cause rails is a jerk ;)
- Remove `method` attribute being required on a `form` for `remote-form`, as rails specifies method with a hidden field `_method` and does `GET` or `POST` anyway
- Serializes forms submitted with `TurboGraft.Remote` in a hopefully much smarter manner

TODO:
- [ ] put a breaking change notice in release notes
Why not allow all types? It works in XHRs.