Change AppCache from browser whitelist to blacklist

Implemented changes talked about in #2235 
In addition to changing AppCache from whitelist to blacklist I also added the discussed change of having Firefox default to 'on', this will break backwards compatibility!

 - What browsers AppCache is enabled in is now using a blacklist system rather than whitelist system.
 - AppCache is now on by default in all browsers.
 - You can use the same config parameters as before to disable AppCache in your browsers of choice.
 - Config option parameter 'browsers' is deprecated, this option was previously undocumented(!)

Pros:
 - Easier to maintain a blacklist than a whitelist. (Example: Opera was unsupported, and therefore off. But it can handle AppCache.)
 - All on by default is more consistent.
 - Less lines, improves code readability.
 - ... ?

Cons:
 - Current developers who rely on Firefox being off by default will have to add 'firefox: false' in config
 - ... ?