Fix close problem on Webrick & speed up

Using rails 3.2.14 and Webrick in development stage, the web server can't close because the call to `exit` in the catching of signals fire an exception on Webrick's code.

Now the close of the connection to Serial port is made in an `at_exit` block as described here:
http://www.bigfastblog.com/ruby-exit-exit-systemexit-and-at_exit-blunder

Also, I've reduced sleep times, so the connection is faster.