Initial implementation of View Manager.

Another PR will be created for the "axiom-shell" repo to use these changes.

Comments welcome.

One thing I don't like is that the vulcanized file are generated "in place" in the "lib" folder. I could not figure out a better way without having to copy the whole "polymer" directory to "out" before invoking vulcanize.

Another unfortunate thing is that the grunt "build" task takes much longer that it used to be because it runs a "bower install" every time.

@rginda 

Sets /seem/ like a better primitive for this kind of dynamic list.  They prevent duplicates, and have first-class remove() and has() methods.  It's also impossible to confuse index and property lookup, and you can safely for..of (once that's on by default) to enumerate them without running into mokey-patched properties.

OTOH, they may turn out to be too slow or awkward to use, in which case we'll switch back to arrays.