Namespaced crypt package #2

This PR adds namespaces to all the Crypt\* classes and implements PSR-4 autoloading.

Squashed version.
I think this is rather unintuitive. You're checking for \phpseclib\Crypt\AES but then you're creating an instance of AES, which might as well be from another namespace. I think it might be better to just use the absolute class name. In general, I think a use statement is not necessary when the class is only ever used once such as in this case.