Implment date (different from datetime) and string in from_numpy

Probably want this to be utf-32 ('U32') rather than utf-8, consider:

```
In [28]: x = np.array(['안녕!'])

In [29]: nd.array(x, dtype='string')
Out[29]: 
nd.array(["\uc548\ub155!"],
         type="strided * string")

In [30]: nd.array(x, dtype='string[3, "U8"]')
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-30-2ec99b294ccc> in <module>()
----> 1 nd.array(x, dtype='string[3, "U8"]')

C:\Users\mwiebe\Anaconda\lib\site-packages\dynd\_pydynd.pyd in dynd._pydynd.w_array.__init__ (dynd._pydynd.cxx:6907)()

RuntimeError: Input string is too large to convert to destination fixed-size string

In [31]: nd.array(x, dtype='string[3, "U32"]')
Out[31]: 
nd.array(["\uc548\ub155!"],
         type="strided * string[3,'utf32']")
```