Implemented spark-splitter

@terrylucas @ikarienator @dinhviethoa This implementation has been loosely ported from polymer-ui-elements, with a number of significant changes in how events are tracked, how event listeners are registered, etc. The HTML/CSS side of things is mostly intact. The way the element is used is also exactly as in polymer-ui-elements, which is different from the up-to-date non-Polymer Spark UI: it uses explicit flexbox and splitter element at the callsite. Actually, this splitter element is exactly the new custom element. I'm thinking about whether a more incapsulated implementation is warranted, like the one in the non-Polymer UI. I think that could be easily achieved as a second level Polymer custom element which would use the one I've just written internally.