API `/v1/system/revision.json` should return block hash of recent block

[msgilligan]

The JSON currently being returned looks like this:

{
    "last_block": 469174,
    "last_parsed": "Thu, 01 Jun 2017 06:46:25 GMT"
}

It would be nice to have the last_block_hash and maybe a block time.

The README has some old information for this call that should be removed.

[msgilligan]

Here's a PR for the README update: #1506