Bring Facebook SDK to PSR4

This updates the code to match the current PSR4 standard in terms of code organisation and autoloading.

I am vastly aware that such a change *cannot* be made against the latest stable version but this repository only has a master branch.
However, I'd really like to stress how *important* this is for modern PHP developers. Class sitting in the global namespace are usually a pain to deal with because no other class can ever be defined bearing the same name (such as a facade per example).

Namespacing the classes is not only good practice but delegates handling of name conflicts to userland where it can be dealt with as the user sees fit.

This pull request is PHP 5.3+. PHP 5.2 is **not** to be maintained anymore and 5.3 itself will be considered deprecated in the near future. So backward compatibility with older PHPs shouldn't be used as an argument here.

This is a really important change. I hope you guys will consider it.