[assets] Generate hashed assets to ensure cache busting on cdns

[dignifiedquire]

When running gulp build --type production it should generate

app-<hash>.js
app-<hash>.css

and the elixir process needs to pic up those names to insert them correctly into the templates

cc @josevalim

[josevalim]

I will take this out of v0.10 milestone because it has worked like this since ever, so it can wait a bit more. In any case, @dignifiedquire if you change gulp in a branch to emit app-<hash> and send a pull request, I can take care of the rest. :) So I am assigning this to you until a PR is sent, then we can move it to me. ;)