Add networkTimeoutSeconds

[gauntface]

See: GoogleChromeLabs/sw-toolbox#221 for details

[jeffposnick]

This only makes sense when used with the NetworkFirst runtime handler, so I'll add it in there directly. (As opposed to adding in somewhere in the generic RequestWrapper interface.)

[gauntface]

It would be good to capture Mat's comment in a test (i.e. if we set timeout of 1 second and request a test endpoint with a 4 second delay while we have a cached response, make sure we get the cached response within, say, 2 seconds and not some time after 4 seconds).