isinstance doesn't work with interfaces

eg

``` python
from System import String, IComparable
s = String("")

# doesn't work
isinstance(s, IComparable)
>> False

# works
s.GetType().GetInterface("IComparable")
>> <System.RuntimeType at 0xaccb048>
```

I think it just needs tp_bases to be set in the typemanager instead of tp_base.
