Allow connection timeouts to be configured (and set 5s default)

Default changed to 5s: This is a reduction from the default timeouts for both Net::HTTP and HTTPClient, but should be more appropriate for Pusher API requests which should complete in << 5s.

em-http-request timeout was set incorrectly previously.

Closes #44
Catch ECONNREFUSED: not caught by HTTPClient
Improve error message when em-http fails