BUG/ENH: Passing explicit dtype should delegate to appropriate Index subclass (if available)
e.g. here's wrong behavior on current master

``` python
In [1]: import pandas as pd
In [2]: ind = pd.Index([1, 2, 3], dtype='int64')
In [3]: ind
Out[3]: Index([1, 2, 3], dtype='object')
```

Problem is ordering of how passed dtypes work in Index constructor.  Make sure this gets fixed in 0.14.
