AES: Fix broken CFB and disableContinuousBuffer()

Related to: https://github.com/phpseclib/phpseclib/pull/45
Using mcrypt's (n)cfb mode now better... 
as Terra pointed it out.

Speedtest:
```php
define('CRYPT_AES_MODE',CRYPT_AES_MODE_MCRYPT);
$aes = new Crypt_AES(CRYPT_AES_MODE_CFB);
$aes->enableContinuousBuffer();

for($part_max_len=1;$part_max_len<=1000;$part_max_len*=10)
	{
		$text = str_repeat('a',10000*$part_max_len);
		for($j=0;$j<3;$j++)
			{
				echo strlen($text)." bytes, randomly partitionated (between 1 and $part_max_len bytes each partition)... ";
				$s=microtime(true);

				for($i=0,$len=strlen($text);$i<$len;)
					{
						$part_len=mt_rand(1,$part_max_len);
						$aes->decrypt($aes->encrypt(substr($text,$i,$part_len)));
						$i+=$part_len;
					}

				$e=microtime(true);
				echo "Time: ".round($e-$s,2)."\n";
			}
		echo "\n";
	}

```

Results with the current code:
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.27
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.29
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.27

100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 2.11
100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 2.1
100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 2.11

1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.92
1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.9
1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.95

10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.93
10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.93
10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.9

Results with  the pull-request code:
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.08
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.08
10000 bytes, randomly partitionated (between 1 and 1 bytes each partition)... Time: 0.07

100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 0.24
100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 0.21
100000 bytes, randomly partitionated (between 1 and 10 bytes each partition)... Time: 0.21

1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.07
1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.08
1000000 bytes, randomly partitionated (between 1 and 100 bytes each partition)... Time: 5.09

10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.87
10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.85
10000000 bytes, randomly partitionated (between 1 and 1000 bytes each partition)... Time: 6.86

(made on win7, php-5.4.8)