Add id of current menu_link to field names.

Fixes a bug which prevented attributes on any item other than the
first being updated, as form field names were being output as:

page_menu[links_attributes][title_attribute]
rather than
page_menu[links_attributes][1][title_attribute]
Merge pull request #22 from skipchris/fix_bug

Add id of current menu_link to field names. Also fixes #19.