Streamlined and unified opacity methods

Currently methods for getting/setting opacity have different workflow:

- in IE w/o opacity support `getStyle_IE` invokes `getOpacity_IE`
- in other browsers `getOpacity` invokes `getStyle` (doing unneeded operations like `normalizeStyleName('opacity')`)

Also `setOpacity_IE` and `getOpacity_IE` check `STANDARD_CSS_OPACITY_SUPPORTED` each time they are invoked (not good for animation performance).
Merge branch 'patch-23' of https://github.com/victor-homyakov/prototype into master-w-updates