Fix nonsecure content warning in IE7 with HTTPS

typeahead.js causes a mixed content security warning in IE7 via HTTPS due to the use of a data url to add a background image (IE7 does not support data urls).

Replacing the data url with about:blank appears to work fine, which I did after reading some background info on a similar workaround in jqueryui. See jqueryui bug #7233 for some background info.

Automated tests are passing after this change, and I've manually tested in IE7, 8 and 9. Having said that, my css and js skills are very rusty, so some further testing wouldn't hurt :-)

Other browsers should not be affected as this in an IE specific workaround.