DataMatrix constructor ignores dtype argument if data is a DataMatrix

```
>>> a = DataMatrix([[1.0,2.0,3.0],[4.0,5.0,6.0]], range(2), range(3))
>>> b = DataMatrix(a, dtype=int)
>>> b.values.dtype
dtype('float64')
```

Not sure if this was a design decision or not, but it seems like the dtype of b.values should be int.
