Set timeout for urb.req
Set timeout for urb.req

This sets a 1 minute timout on the urb.req xhr object. If no response
is received after one minute, it passes an error to the callback with a
408 status code.

I've also made sure that a 'beat' response resets the dely property back
to 250, instead of maintaining the previous value.

Fixes urbit/talk#21

belisarius222/arvo,urbit/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,jfranklin9000/urbit,eglaysher/arvo,urbit/urbit,ngzax/urbit,ohAitch/arvo,urbit/urbit,cgyarvin/arvo,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ohAitch/arvo,joemfb/arvo,jfranklin9000/urbit,Fang-/arvo,urbit/urbit,solri/arvo,ngzax/urbit,jfranklin9000/urbit,joemfb/arvo,belisarius222/arvo,jfranklin9000/urbit,urbit/urbit,cgyarvin/arvo