Move code from digitallyhappy/assets to backpack/bassets

[tabacitu]

Let's make this a self-standing repo. To have that, we should:

• move all the code from this assets branch to this repo.
• change the composer.json for this new package
• we DO NOT add the package to Packagist yet, we'll keep it closed-source; until we make this public, to use it in our apps (the Demo most likely) we can add it to https://repo.backpackforlaravel.com/ - Antonio just tell me when it's ready and I can do that real quick; then Composer will pull it just like it does from Packagist;
• change the README to detail this new package, instead of loadOnce and other stuff that DigitallyHappy/assets offered;
• remove loadOnce() and other functionality that is no longer needed in this package; people should be able to install Bassets and DigitallyHappy/Assets side-by-side if they still use @loadOnce, but this package should be a COMPLETE replacement; there shouldn't be any use cases where DigitallyHappy/Assets is still needed, other than backwards-compatibility because you're too lazy to change your code;

[tabacitu]

I think it's just easier for ME to do this, because I want to keep all change history and think about the docs a little bit. So when all the features inside digitallyhappy/assets:bassets are ready... I'll do this.