Hide new record link for has_many form

This changes will do:
1) add option 'new_record' for has_many form to hide 'new item link'
2) add missed specs for 'heading' option that discussed in [#1945](https://github.com/gregbell/active_admin/pull/1945)
3) add changes to documentation to docs/5-forms.md

Screenshots to illustrate changes:

Showing Add button
![before2](https://f.cloud.github.com/assets/583275/204449/66f1de62-8179-11e2-9846-a6afc0084001.png)

not showing Add button
![after2](https://f.cloud.github.com/assets/583275/204446/66c091ea-8179-11e2-9de3-92e265d62c4a.png)