Support for HDF5 format in Storage class

This implements preliminary support for HDF5 format in Storage class.

Also, I think that, as HDF5 allows to store several datasets inside a file, the Storage constructor would require a new parameter to specify the dataset name inside the file.  I suggest using a new, optional, `datapath` parameter for that.

One of the tests fails due to
https://github.com/ContinuumIO/dynd-python/issues/51