[MERGE] Add Hooks.install_named_hook

[Robert Collins]

Robert Collins has voted approve.
Status is now: Approved
Comment:
Alexander - just altering the signature of install_hook is  aproblem for 
plugin authors, because they cannot tell if they can pass in a name or 
not. so its cleaner to rename the function.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20080310185219.4b2c0276%40lapbert.oxbridgetech%3E