Renamed this element to iron-splitter and ported to Polymer 1.0

[JC-Orozco]

I ported the core-splitter element to Polymer 1.0 and renamed it iron-splitter.
I am aware that this elements reside now on PolymerElements but since there is no iron-splitter element yet I decided to try a push request here to find advice.

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[JC-Orozco]

I signed it!

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