No longer use once() for events in putTile().

It is too easy to miss an `error` event due to uncertainties about the
node http api. Use a guarded callback instead.
Abort request after timeout.

http://nodejs.org/api/net.html#net_socket_settimeout_timeout_callback
https://github.com/joyent/node/blob/master/lib/http.js#L1380-L1390
Add error handler to response object.
Merge pull request #5 from mapbox/hangup

Fix socket hangups