Drop support for IE 6-8 according to the death of jQuery Compat 3.0

[flowersinthesand]

Cettia JavaScript Client's browser support policy has been the same with the one of jQuery 1.x, and the one of jQuery Compat 3.0 was supposed to replace the one of jQuery 1.x. However, as jQuery team announces the death of jQuery Compat, now the browser support of jQuery 3.0 should be used so we don't need to support IE 6-8. (FYI, I thought it's a waste of time to try to support browsers where jQuery doesn't work)

The following fallback can be dropped:

• util.createXMLHttpRequest for XMLHttpRequest
• util.parseJSON and util.stringifyJSON for JSON
• createHttpLongpollJsonpTransport for script.onreadystatechange
• head for document.head
• util.isArray for Array.isArray
• Specific fallbacks for IE 6

Unfortunately, IE 9's XMLHttpRequest is not capable of cross origin operation so there will be no changes in transport factory.

[flowersinthesand]

FYI, a64edb9 reduced the size of cettia.js a lot.

Before

Original Size: 11.03KB gzipped (41.82KB uncompressed)
Compiled Size: 5.18KB gzipped (14.27KB uncompressed)

After

Original Size: 9.81KB gzipped (37.53KB uncompressed)
Compiled Size: 4.43KB gzipped (12.31KB uncompressed)

[flowersinthesand]

Now ECMAScript 5 can be used safely.