use a submodule for core files

[cfoellmann]

I would like to suggest to "outsource" the https://github.com/aws/aws-sdk-php/tree/master/src/Aws folder to a submodule to make it possible to use the core files in custom applications.

I know it would be a major restructuring but would make a lot of things much easier.

[mtdowling]

Removing files from this repository and replacing them with git submodules would be a major breaking change for users who are installing the SDK using git and would add a great deal of overhead to maintaining the SDK. I recommend using Composer to manage your dependencies and bring the SDK into your projects as needed.

We offer a number of alternative ways to install the SDK: Composer, PEAR, GitHub, Phar.