Namespaced crypt package #2

This PR adds namespaces to all the Crypt\* classes and implements PSR-4 autoloading.

Squashed version.
> Considering that a missing leading slash could be interpreted as a relative namespace by readers, I think there should always be a leading slash.

Yes I can see that. The only thing is that other methods would not return the slash at the beginning, get_class returns "phpseclib\Crypt\AES" for example, so we might have a consistency problem. Of course, those should eventually be replaced by instanceof calls, so it might not be an issue long term... Just let me know which way to go and I will make it happen.