Added support for assets outside Escher pkg

[mdcfrancis]

Added support for Escher assets outside of the main Escher code base. You can now write

push!(window.assets, ( "<pkg>", "<asset>"))

which will load the asset from

Pkg.dir( <pkg>, "assets" ) 

Polymer components can also reference asset dependencies as, this allows the use of additional libraries

<script type='text/javascript' src='/pkg/<pkg>/<script>.js'></script>

I've also fixed up a few of the v0.4 warnings.

[rohitvarkey]

This is super cool! :+1: