$.ajax cache: false edge cases

[kswedberg]

check with @jaubourg before updating

• mention that cache: false works only with HEAD and GET requests,
• mention that it does so by adding/replacing a "_={timestamp}" to the get parameters
• the param is NOT needed for other requests except in IE8 when you POST to a URL that you already requested with a GET

[jaubourg]

Seems good, Karl.