Namespaced crypt package #2

This PR adds namespaces to all the Crypt\* classes and implements PSR-4 autoloading.

Squashed version.
Technically, these use statements are not required, as e.g. "Base" will be resolved to "phpseclib\Crypt\Base" due to the "namespace phpseclib\Crypt;" declaration. I am fine with having them as long as this is consistent with your previous PRs. What do you think?