Add instructions for strong params gotcha

Instructs user to explicitly permit the _delete parameter cocoon relies on for nested objec deletion
Catch typo of `_delete`. Should be `_destroy`
Small changes to the wording of strong parameters gotcha.

- strong parameters can also be used as a gem in rails3
- the `_destroy` flag is not specific to cocoon, but
  rather to nested model handling in rails in general.
Fix deprecation warnings in dummy app
Remove temporary objects from association after their creation
Adapt README: `:force_non_association_create` is rendered obsolete.

I hope. I will not remove it yet for backward 
compatibility reasons. 
Not sure if somebody actually needs an object to be
_not_ created on the association. Maybe for 
other ORM's?
Add `:form_name` parameter. Fixes #153.

This gives users the option to use a different
form parameter than the default `f`.

Usage:

    link_to_add_association 'add something', form, :people, :form_name => 'person_form'
Adapted history.
Remember destroyed state after validation errors. Fixes #136 and #160.

The `link_to_remove_association` correctly keeps the `_destroy`
flag, and gets a class `destroyed` which is used to automatically
hide the nested element.
Version bump to 1.2.1
Regenerate gemspec for version 1.2.1
Explain required elements

Took me a little debugging effort to discover the css/html wrapping requirements. Something like this would have helped me a tad.
Make cocoon more lenient to has_one null etities.