[Proposal][WIP] Restructuring files

_This is juste an example of how to restructure files and keeping backward compatibility until the next major release._

The SDK is growing up and now really use namespaces (mostly thanks to @SammyK). But there are classes that should be namespaced like exceptions, helpers and graph objects but there aren't because of backward compatibility. I propose to move them to there own namespace and deprecate old ones until the next major release.

And as composer is the official way to use the SDK, we can use its `classmap` functionnality to load a file with all the deprecated classes.

This POC moves only helpers.

@gfosco, any thought on this ?