Easing functions

[Emmanuel Bourg <ebourg@apache.org>]

Hello,

Thank you for applying my latest suggestions Kirill.

I looked around at the other animation frameworks available and I 
noticed several easing functions that are quite common. Things like 
'bounce', 'elastic', 'back' with variants for both ends (in, out, in and 
out).

Do you think these functions could be part of the core Trident API? This 
is something I'm interested in working on.

Emmanuel Bourg