Changed default Twisted DNS Resolver to be blocking

The default Twisted reactor uses a threaded DNS resolver by default (at least on platforms that support threading).  As a result, if you're using a Twisted library, and you're looking up hostnames, all of the sudden your single threaded Tornado server will spawn additional threads.

This is kind of annoying because
1)  Signals like Ctrl+C no longer work for the main Tornado process because the worker threads will be blocked on their request queues.
2)  It's inconsistent with other Tornado functionality such as AsyncHttpClient which for better or worse, uses blocking dns lookups.
3)  It's confusing - I dug through stuff for hours convinced I wasn't integrating Twisted + Tornado correctly.

So, I changed the default to make my life easier, but I can totally understand if you reject this, given that the Twisted behavior is both correct and more efficient.  But, I figured I'd try to save others a few hours of time in case they suddenly get confused when Ctrl+c stops working in a Twisted+Tornado app.