Improve assets versioning method #16

Current assets versioning implies, that any change in assets will result in new path for every asset, even if most of them was not changed, e.g. we have:

https://assets.myapp.com/v1.0.0/main.js
https://assets.myapp.com/v1.0.0/lib-a.js
https://assets.myapp.com/v1.0.0/lib-b.js <- we change this one

now we have:

https://assets.myapp.com/v1.1.0/main.js
https://assets.myapp.com/v1.1.0/lib-a.js
https://assets.myapp.com/v1.1.0/lib-b.js

And the problem is that cache for assets that was already downloaded from v1.0.0 will be useless, now browser will download their copies from v1.1.0

The better method could be adding version for individual assets (like most of JS libraries do):

https://assets.myapp.com/main-v1.0.0.js
https://assets.myapp.com/lib-a-v1.0.0.js
https://assets.myapp.com/lib-b-v1.0.0.js <- we change this one

https://assets.myapp.com/lib-b-v1.1.0.js <- now we have only 1 asset changed, all other cache is kept