Useful repr for xblock state

@doctoryes @nedbat Seven line commit adds a useful repr to workbench/models.py. Makes it more convenient to debug in a Python shell. 
FYI, I find this a little nicer:
```
return (u"<XBlockState id={self.id} "
    "scope={self.scope} "
    "etc..."
    "tag={self.tag}>".format(self=self))
```