Base/AES/Rijndael: Optimizations

- Base/Rijndael/AES: Comments updated.

- Base: removed __desctructor(). Re:
https://github.com/phpseclib/phpseclib/issues/107

- Base: setPassword() will use >= php-5.5's new (faster) hash_pbkdf2()
function if availalbe/possible. 100% compatible to the internal
implementation. Strong testet with all
hashes/iterations/lengths/ciphers.

- Rijndael: Runtime engine-switch: Will now use mcrypt (in case of
128/192/256-bit block/keys), if possible. Otherwise MODE_INTERNAL.

AES: Soucecode reducing. After all, now, AES.php is virtually nothing
other than a wrapper to Rijndael.php::new
Crypt_Rijndael()->setBlockLength(128). No different in speed or
functionality, but fixed block_size.