Re-think Validators, add messages and minor fixes

- Separates valdators into a `SirTrevor.BlockValidations` mixin.
- Add the validators to the `SirTrevor.Block` by default.
- Removed any length check validations from SirTrevor, these will now have to be a custom validator.
- Adds a new method for defining custom validators (as functions).
- `Editor` now calls `valid` instead of `validate` on `performValidations`
- Logs errors on a field per field basis with messages.
- Adds a new $messages container at the top of the block
- `addMessage` function displays a message in the messages container
- Stop us binding everything on the `_bindFunctions` method, only bind the `this.bound` 
- Shows an error on incorrect parsing of JSON now
- All borders are now set as pixel widths to maintain consistency