Determine the active children on render

If I apply *shapeshift* to a container with children, then remove some of its children and add new ones, and trigger a `ss-rearrange` on the container, I expect the new children to be shapeshifted correctly.

However, triggering `ss-rearrange` calls render, which currently does not recalculate the children of the container, so the new nodes are not taken into account.

My solution involves calling `setActiveChildren` on render if `reparse` is `true`, which activates and parses new children, and makes everything work, but I'd like to hear your opinion on the performance implications of this.
Merge pull request #52 from unindented/patch-1

Determine the active children on render