Add support for asset host

[brenogazzola]

Closes #104

I'll leave this unmerged for now so others can test and give suggestions

[dhh]

I'm not sure why this is actually still needed? If we're outputting relative URLs, wouldn't those be relative to wherever the CSS is already being served from?

[brenogazzola]

This was requested for users whose assets are not in the app's domain, but other domains, like S3.

This is usually used by apps that perform rolling releases and have a lot of servers. For example, if you change application.js and a user refreshes a page, the HTML request might go to server web1 which has been updated and will point to application_new.js, but the JS request might go to server web13 which has not been updated yet (because the rolling release is still on server web8) and therefore does not have that file, only application_old.js. This will cause a 404. Less of a problem with images, large problem with application.js in an SPA.

[brenogazzola]

If you configure your app's asset_host to point to your CDN and upload the new assets to there before you start a rolling release, the problem is solved.