Enable support of retina displays

[1scaR1]

It seems to be ok to enable retina support.
I don't why it was disabled at all...

[pandamicro]

Actually this macro is never used and never set, cc.view.isRetinaEnabled() is the correct API, we haven't cleanup these macros

[1scaR1]

It's actial use in cc.contentScaleFactor function. If this macro is set to 0 then this functions always returns 1.
In actual code it breaks compability with jsb on retina displays, because 'setContentScaleFactor' not works.
If you want to remove this macro my propose is to set cc.contentScaleFactor always to return Director's _contentScaleFactor in this PR. Is it right?

[pandamicro]

If you want to remove this macro my propose is to set cc.contentScaleFactor always to return Director's _contentScaleFactor in this PR. Is it right?

I think it would be more proper

[1scaR1]

@pandamicro Done! Please, review

[pandamicro]

Thanks～