Change the default search engine in TBB

Some users are concerned that the default search engine in TBB is Google. While we do rewrite the parameters sent to Google, the fact that google can see a large percent of the tor user search queries (as a whole community) may be reducing their anonymity set. The power of defaults means many users, perhaps the majority, will not change their behavior.

Alternatives to google by default are ixquick and duckduckgo. My vote is for ddg. They run a hidden service, exit enclave, and in general seem to support Tor. ixquick has refused to work with us in the past.