Add the option 'delay' in order to reduce the number of request to the server while typing

I added a numeric option called 'delay' (with a default value of 250ms), so that when you're typing, let's say "foo bar", only one single request is sent to the server (assuming you type fast enough :)), not 7 as it was working until now. It is disbled with the value "0" (actually with any non-positive value).

Just in case, I read all the calls to _lookup_ and the returned value was not being used anywhere. The documentation says ".lookup: To trigger the lookup function externally", it doesn't say it returns anything useful so I guess it's ok...