New PHP >= 5.3.0 implementation of phpseclib

I made a couple of small modifications to the main fork to adjust PHPSecLib for my projects that use PHP >= 5.3.0.

These modifications consist of :

* Addition of namespace `PHPSecLib` to all files
* Replacement of function `crypt_random_string` with class `Crypt_Random`, with an abstract function named `string`
* Replacement of all constructors with a `__construct` function and references to them
* Deletion of all `!class_exists(...){ include ... }` segments to allow more flexible autoloading
* Delection of the `_is_includable` function and references to them
* Addition of an autoloader fully compatible with the PSR-0 standard
* Addition of `public` and `protected` to methods

See issue https://github.com/phpseclib/phpseclib/issues/369.

**EDIT :**

I updated namespace `PHPSecLib` to `phpseclib/phpseclib`