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. 
works for all google domains, including google.co.uk, google.com.cn etc.