Add Array Member Type

That was quite fun, thanks for putting up with me.

```python
from atom.api import Array, Atom
import numpy as np

class Test(Atom):
    arr1 = Array()
    arr2 = Array((1,2,3))
    arr3 = Array([1,2,3])
    arr4 = Array([1,2,3], kwargs=dict(dtype=np.float64))
 
t = Test()
assert t.arr1.size == 0
assert t.arr2.size == 3
assert t.arr3.dtype == np.int
assert t.arr4.dtype == np.float64 
```