Reject promise on request timeout

Firefox and Chrome trigger the request's `onerror` handler when a
timeout occurs. Safari triggers the `ontimeout` handler.

This can be observed by making a request to an unroutable address:

    var xhr = new XMLHttpRequest()
    xhr.onload = console.log.bind(console, 'loaded')
    xhr.onerror = console.log.bind(console, 'errored')
    xhr.ontimeout = console.log.bind(console, 'timeout')
    xhr.open('GET', 'http://10.255.255.1')
    xhr.send()

Fixes #294.

fetch.js

@@ -368,6 +368,10 @@
         reject(new TypeError('Network request failed'))
       }
 
+      xhr.ontimeout = function() {
+        reject(new TypeError('Network request failed'))
+      }
+
       xhr.open(request.method, request.url, true)
 
       if (request.credentials === 'include') {