Make backreferences hideable

Added `boolean` field "Show backreferences". Default: `True`

# Screenshots

Default state "Show backreferences" checked. Like before.
![screen shot 2014-11-27 at 12 00 29](https://cloud.githubusercontent.com/assets/1375745/5215979/4c751d60-762d-11e4-99e2-f1c6c9c89708.png)

Edit form.
![screen shot 2014-11-27 at 12 01 16](https://cloud.githubusercontent.com/assets/1375745/5215987/5924fe4a-762d-11e4-8a15-28b52661e990.png)

Disabled.
![screen shot 2014-11-27 at 12 01 25](https://cloud.githubusercontent.com/assets/1375745/5215989/65de3462-762d-11e4-9770-b12ab200e332.png)

@maethu