Namespaced crypt package #2

This PR adds namespaces to all the Crypt\* classes and implements PSR-4 autoloading.

Squashed version.
There are other class_exist calls missing the leading slash, so we can alternatively handle this is a follow-up PR.
```
grep class_exists . -Ri | grep phpseclib
./Math/BigInteger.php:        if (class_exists('phpseclib\Crypt\Random')) {
```