Drop support for old browsers

[flowersinthesand]

Support set will be:

• Internet Explorer: 6+
• Chrome: (Current - 1) or Current
• Firefox: (Current - 1) or Current
• Safari: 5.1+
• Opera: 12.1x, (Current - 1) or Current

This set is the same with one of jQuery 1.x.
http://jquery.com/browser-support/

The followings will be dropped :
Chrome
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L1445-L1446 - Chrome 10-15 is not supported
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L242 - so that browser.chrome can be removed

Firefox
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L253-L255 - Firefox 3 is not supported
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L1316 - Fifrefox 6 is not supported
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L246 - so that browser.mozilla can be removed

Safari
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L243 - browser.webkit can be removed

Opera
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L942-L947 - works correctly in 12.1
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L1497-L1499 - not tested with 12.1 but sse is supported
https://github.com/flowersinthesand/portal/blob/1.0.1/portal.js#L244 - so that browser.opera can be removed

[flowersinthesand]

Confirmed that Opera 17.0 don't need to use work around in streamxhr. But 17 is not minimum version for that.