RFC: Dataset storage information in HDF5/JSON

This is a proposal to add dataset storage information to HDF5/JSON. JSON key for this is named `byteBlocks`. The word "block" is hopefully still technically accurate while not too similar to "chunk".

Below is an example for one dataset. Block JSON keys are in the same format as in the HSDS schema, e.g. `0_0` and `0_1`. Two blocks are in the example; the `0_1` block has an optional `url` key in case of remote blocks. The `url` key can also apply to the entire dataset, in which case cannot appear in the blocks.

```json
{
"datasets": {
"7f335a2e-7ab1-11e4-87a5-3c15c2da029e": {
"attributes": [],
"dcpl": {
"fillValue": 0,
"layout": {
"class": "H5D_CHUNKED",
"dims": [8]
}
},
"shape": {
"class": "H5S_SIMPLE",
"dims": [10, 10],
"maxdims": [10, 10]
},
"type": {
"base": "H5T_STD_I32BE",
"class": "H5T_INTEGER"
},
"byteBlocks": {
"0_0": {
"offset": 1234,
"size": 2567,
},
"0_1": {
"offset": 56789,
"size": 1967,
"url": "s3://mybucket/path/to/object"
}
}
}
}
}
```

cc: @derobins @jreadey @gheber