redirect download to summary page if referer is google/yahoo/bing

files changed:
src/java/edu/psu/citeseerx/web/FileDownloadController.java
tested on csxstaging01
how to test: use google referer controller to change the referer, paste 
http://csxstaging01.ist.psu.edu/viewdoc/download?doi=10.1.1.19.9967&rep=rep1&type=pdf
to your google chrome browser and see if you can download the pdf file or it is redirected to the summary page.