Add support for Swift Package Manager

[Daboo1998]

I would love to use this library, but I want to do it using the Swift Package Manager as I load all third party frameworks using it. SPM is used more and more, which makes integration with it a logical move to do. Please consider it.

[8secz-johndpope]

I'm using this with swift / cocoapods thanks to @eugenebokhan - but can't get animations working...
https://github.com/johndpope/demo-unistroy

[johndpope]

https://github.com/ndreca/Assimp