Expose the adapt type/operation to python

Implemented is adapt operations for int to date/datetime, using netcdf-style metadata.

```
In [1]: from dynd import nd, ndt

In [2]: a = nd.array([1, 3, 10, 31, 365])

In [3]: nd.adapt(a, ndt.date, 'days since 2012')
Out[3]: nd.array([2012-01-02, 2012-01-04, 2012-01-11, 2012-02-01, 2012-12-31],
                 type="strided * adapt[(int32) -> date, "days since 2012"]")

```