Util functions converting formats between HDF5 and Blob

To use the HDF5DataLayer to [provide advanced data set](https://github.com/BVLC/caffe/pull/126#issuecomment-37570755) and to [enable input from various data sources](https://github.com/BVLC/caffe/pull/196#issuecomment-37707842), there should be functions converting Blob from and to other formats.

HDF5 is a very popular data set format in the scientific computing community. Building the bridge between it and Blob will open a lot of new opportunities to solve #126 and many complex practical problems.