BUGFIX for ssh public key fingerprints where there is a comment that does not contain "@"

I found a bug while parsing public key fingerprints.  If there's a comment that doesn't contain "@" then the entire string is decoded and passed to `Digest::KLASS.hexdigest` resulting in an incorrect fingerprint:

```ruby
1.9.3p448 :002 > SSHKey.sha1_fingerprint "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9HuXvYJPtQE/o/7TYi63yAopsrJ6TP+lDGdyQ+nVVp+5ojAIy9h8/h99UlNxjkiFT2YhI3Fl/pgNDRO4PVo6tlgb3CwiAZjSdeE5RnF79Dkj5XsM4j+FLMoXtbRw0K9ok9RKjz6ygIs1JDmaOdXexFnq4nAYU3fSLUa6WoccqTHe8bFuJoAv1gbnx09Js8YcVMD96mpTJ3V/MK5YfIv10dbtrDhGug3IS1V2J+0BB9orbQja554N+4S0I9rFBgVCpvPmQqddDHd/AdGkLv/zjEfGytjnvp68bEfDinkQkPfuxw01yd5MbcvLv39VVICWtKbqW263HT5LvSxwKorR7"
 => "e5:c2:43:9e:e4:0c:0c:47:82:7a:3b:e9:61:13:bd:9c:43:eb:4c:b7"
1.9.3p448 :003 > SSHKey.sha1_fingerprint "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9HuXvYJPtQE/o/7TYi63yAopsrJ6TP+lDGdyQ+nVVp+5ojAIy9h8/h99UlNxjkiFT2YhI3Fl/pgNDRO4PVo6tlgb3CwiAZjSdeE5RnF79Dkj5XsM4j+FLMoXtbRw0K9ok9RKjz6ygIs1JDmaOdXexFnq4nAYU3fSLUa6WoccqTHe8bFuJoAv1gbnx09Js8YcVMD96mpTJ3V/MK5YfIv10dbtrDhGug3IS1V2J+0BB9orbQja554N+4S0I9rFBgVCpvPmQqddDHd/AdGkLv/zjEfGytjnvp68bEfDinkQkPfuxw01yd5MbcvLv39VVICWtKbqW263HT5LvSxwKorR7 foo key"
 => "18:48:25:29:09:bc:be:36:52:47:ae:7e:3d:1e:93:71:91:fd:5a:80"
1.9.3p448 :004 > SSHKey.sha1_fingerprint "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9HuXvYJPtQE/o/7TYi63yAopsrJ6TP+lDGdyQ+nVVp+5ojAIy9h8/h99UlNxjkiFT2YhI3Fl/pgNDRO4PVo6tlgb3CwiAZjSdeE5RnF79Dkj5XsM4j+FLMoXtbRw0K9ok9RKjz6ygIs1JDmaOdXexFnq4nAYU3fSLUa6WoccqTHe8bFuJoAv1gbnx09Js8YcVMD96mpTJ3V/MK5YfIv10dbtrDhGug3IS1V2J+0BB9orbQja554N+4S0I9rFBgVCpvPmQqddDHd/AdGkLv/zjEfGytjnvp68bEfDinkQkPfuxw01yd5MbcvLv39VVICWtKbqW263HT5LvSxwKorR7 foo@key"
 => "e5:c2:43:9e:e4:0c:0c:47:82:7a:3b:e9:61:13:bd:9c:43:eb:4c:b7"
```


I confirmed this fix produces the correct fingerprint regardless of the key comment.  This fix also unifies the public class methods (less `::generate`) by using a single ssh public key parsing technique `::parse_ssh_public_key`.

```ruby
SSHKey.fingerprint "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9HuXvYJPtQE/o/7TYi63yAopsrJ6TP+lDGdyQ+nVVp+5ojAIy9h8/h99UlNxjkiFT2YhI3Fl/pgNDRO4PVo6tlgb3CwiAZjSdeE5RnF79Dkj5XsM4j+FLMoXtbRw0K9ok9RKjz6ygIs1JDmaOdXexFnq4nAYU3fSLUa6WoccqTHe8bFuJoAv1gbnx09Js8YcVMD96mpTJ3V/MK5YfIv10dbtrDhGug3IS1V2J+0BB9orbQja554N+4S0I9rFBgVCpvPmQqddDHd/AdGkLv/zjEfGytjnvp68bEfDinkQkPfuxw01yd5MbcvLv39VVICWtKbqW263HT5LvSxwKorR7  foobar 2013/09/26"
 => "04:1b:d4:18:df:87:60:94:8c:83:8a:7b:5a:35:59:3d"
```

```bash
$ ssh-keygen -l -f foo
2048 04:1b:d4:18:df:87:60:94:8c:83:8a:7b:5a:35:59:3d   foobar 2013/09/26 (RSA)
```


NOTICE:  This does raise an exception if the key is invalid, like `::ssh_public_key_bits` does.  This also requires the sshkey type to be passed ("ssh-rsa/ssh-dss") with the ssh public key, deprecating the ability to pass a string without a `SSH_TYPES` assignment.