reduce filesize and performance optimization

[cyfung1031]

reduce filesize by some code fixes

Remark 1: Reason for changing this.each ( callback ) to each ( this, callback )

For the code minimization, this.each( will be reduced to d(

Remark 2: changing wrapper.children.length to wrapper.firstElementChild

When calling children, it will create a new NodeList that containing all the sub-elements under the element wrapper.
As this is just to check whether there is any element child or not, firstElementChild or lastElementChild shall be used for performance consideration.

Remark 3: .insertBefore ( ele, null ); is the same as .appendChild ( ele );

[cyfung1031]

@fabiospampinato this can reduce filesize without change of any behavior.