add a way to whitelist attributes to serialize.
allow to change the _destroy attribute by setting destroy_action.
allow to change the _destroy attribute by setting destroy_action.
add test for whitelisted attributes.
add some test for destroy_action.
Merge branch 'destroy_action'
Merge branch 'serialize_keys'

Conflicts:
	spec/javascripts/backbone-nested-attributes/ModelSpec.js
add the destroy_action to the attributes whitelist.
document the serialize_keys and destroy_action functionalities.
Merge pull request #14 from igwan/master

Allowing one to whitelist attributes to serialize on a relation and customize destroy action field