Add :allow_destroy option to has_many forms

This option will add a boolean field "Remove"
to the end of each has_many fieldset that is
an existing record. This provides a workaround
for issue #1990, and allows for cleaner form
coding.
Add i18n for has_many_remove label
Add docs for using has_many forms featuring :allow_destroy
Merge pull request #2071 from shekibobo/has-many-allow-destroy

Add :allow_destroy option to has_many forms