Add support for deleting a single IP

[michaeljs1990]

This lets you specify the IP you would like to delete from an asset without needing to know the pool of that asset. Stange enough this is already documented in the API however it was not implemented.

http://tumblr.github.io/collins/api.html#api-ipam-update-an-address

[michaeljs1990]

Heh, turns out two years ago someone else wanted to do this also #505 was recently closed and adds this.