Fixes distribution of item template via named slots (Fixes 479)

[cailyncodes]

This PR fixes #479 by having each stamped instance of an item template inherit the slot attribute on the template item. In the _createPool function, the template is stamped and each new instance is appended as children to the template's parent node. When the template reaches the iron-list element via named slots, this will result in an incorrect distribution of the new nodes, because iron-list will not be able to correctly identify its assigned nodes.

See #479 for reasoning behind this approach over modifying the behavior of Polymer.Templatizer and a work around until this lands.

The accompanying tests use two levels of sloting to demonstrate this.