Simplify code

see http://docs.python.org/2/library/uuid.html
and python2.7 uuid.py

``` python 
def __str__(self):
        hex = '%032x' % self.int
        return '%s-%s-%s-%s-%s' % (
            hex[:8], hex[8:12], hex[12:16], hex[16:20], hex[20:])

....

def get_hex(self):
        return '%032x' % self.int
hex = property(get_hex)
```