Cleanup effects.js to improve performances and reduce filesize

[lrbabe]

I've opened a  pull request of small changes to effect.js that:

• reduce function calls, jQuery.now() calls in particular
• reduce number of out-of-scope variables lookups
• cache more variables to reduce file-size once minified

It yields some interesting performance improvement, as demonstrated in this example:  http://jsfiddle.net/louisremi/V3Xqt/5/ (remove patched effects.js in "manage ressources" menu to see the difference)

• function calls: 1177 -> 1022 = 13% better
• average execution time: 26.1 -> 23.4 = 10% better

/!\ example might be unrealistic /!\

As a side effect, it keeps animations synchronized and thus fixes #7917 and addresses a similar problem to #6281

[lrbabe]

Additional benefits with last commits:

• reduce function calls even more
• simplifies easing resolution code
• add some comments and improve code readability here and there

[john]

This particular pull request was closed - a new open was opened up (and closed) just recently.