running add,set,remove,move in the Observation update queue

#56

This fixes an issue where parent.replacements was null at times in the register function in cases where an {{#if ...}} surrounded an {{#each ...}} and the list was modified in the same batch as the if was toggled to remove the each:

{{#if show}}
  {{#each items as item}}
    {{item}}
  {{/each}}
{{/if}}

show = false;
list.replace([ 'two', 'three' ]);