LazyLoad: Add params for timeout period and timed-out template

[bphelan]

It would be nice to have two more snippet params added to LazyLoad: one to specify the CometActor timeout and another to specify a second template to use in the case of a timeout.

[github-importer]

Imported from Assembla: http://www.assembla.com/spaces/liftweb/tickets/1098

[Shadowfiend]

Notably, Lift 3 supports a template parameter that can be passed to the lazy snippet. It doesn't support customization of the timeout, though. We'd need to support that directly in AsyncRenderComet, and then allow for setting it in the LazyLoad snippet.