Panel constructor ignores dtype
I thought this was fixed(see issue 411), but it appears to keep resurfacing ;)

In [27]: a=Panel(items=range(2),major_axis=range(10),minor_axis=range(5),dtype=np.float32)

In [28]: a[0].dtypes
Out[28]: 
0    float64
1    float64
2    float64
3    float64
4    float64

A quick debug shows that the routine form_blocks just considers generic "float", "int" etc datatypes, so maybe it does not distinguish between float32 and float64?
