Support temporary password saving

[GoogleCodeExporter]

Currently the master password can only be stored forever or not at all. It 
would be nice to support storing it for a fixed length of time or until the 
browser is closed.

Storing it for some fixed time safely would require a background page, which 
the extension doesn't currently have and would make it take more memory. 
Storing it until the browser is closed seems like it would be possible with 
HTML5 session storage, but currently every extension page invocation (i.e. each 
new popup) has a fresh HTML session key and thus session storage doesn't 
persist.

Original issue reported on code.google.com by [email protected] on 10 Aug 2012 at 11:25