Add a page:received event.

For a page load that is not from the Turbolinks cache, this event
callback is triggered after page:fetch but before page:change,
specifically as soon as the XHR response body is available (in the
XHR onload handler).

Having an instrumentation point as early as possible once a response is received
from the server allows for a more accurate measurement of network vs. frontend
time when Turbolinks is enabled. This is important for performance-monitoring
tools like New Relic (which, full disclosure: I work there).

Note: it might also be useful to attach the actual XHR object to this event, in
order to allow client code to inspect and/or modify it before it's seen by the
rest of the Turbolinks code, but lacking a concrete use case, I opted to skip
that for now and keep this as minimal as possible.