$('body').append() will never execute script.

**Note: this might not be a bug, if you think it is a good idea to execute script in $(selector).append() just as jQuery does.**

### bug decription
the script which is appended to the DOM will never be executed.

### how to reproduce?
try this
```
af('body').append('<script>alert(1)</script>');
af('body').append('<div>this is newly added.</div><script>alert(2)</script>');
```

### how to fix?
the code below never get a chance to run.
```
if (obj.nodeName != nundefined 
    && obj.nodeName.toLowerCase() == "script" 
    && (!obj.type 
    || obj.type.toLowerCase() === 'text/javascript')) 
{
    window['eval'](obj.innerHTML);	
}
```
**because the obj is either a $afm instance or undefined, so the obj.nodeName will always be undefined.**
so I just changed the code logic to make it works, check it out.

a little more advice: if you do think it is really not a good idea to support this feature due to the performance of af, then just discard it.
Merge pull request #498 from LeoYuan/patch-5

$('body').append() will never execute script.