Share common functions between bezier functions.
Share common functions between bezier functions.

* That way the constructor is faster (16% faster aprox, measured in
  http://jsperf.com/bezier-easing-benchmark).
* Also, since all bezier curves share memory references to the same
  functions instead of having its own copy, it will save memory,
  although I haven't measured how much it saves.
* The execution times are identical. Measured in
  http://jsperf.com/bezier-easing-execution

myfancypants/bezier-easing,gre/bezier-easing,JoeKeikun/bezier-easing,archiveds/bezier-easing,maxxfrazer/bezier-easing,myfancypants/bezier-easing,gre/bezier-easing,maxxfrazer/bezier-easing,alethes/bezier-easing,alethes/bezier-easing,gre/bezier-easing,jinlongwang/bezier-easing,archiveds/bezier-easing,JoeKeikun/bezier-easing,jinlongwang/bezier-easing