Cleanup module

Hi,

This PR slightly clean up the module:

- I couldn't run the tests because it could not load the vendor/autoload file. I've made it a little more robust, based on what we did on DoctrineModule.
- This is a little BC but I think this module should not be under namespace Aws but AwsModule (as it is for DoctrineModule, DoctrineORMModule, for the only reason we may have clashes).
- I removed the closure in Module.php and replaced it by a factory instead, it's a bit more flexible as people can eventually override it so they can introduce their own things when creating an AWS client (well, closure can be overloaded too but I find it strange to introduce object creation in Module.php, it's clearer in a separate file).

Thanks ;-).