Add more API endpoints about dcrdata

[chappjc]

This should close with a /api/v1

[chappjc]

With PR #25, added:

/mempool/sstx
/mempool/sstx/fees
/mempool/sstx/fees/N
/mempool/sstx/details
/mempool/sstx/details/N

[chappjc]

With PR #35, added:

/stake/pool
/stake/pool/b/X
/stake/pool/r/X/Y
/stake/diff/b/X
/stake/diff/r/X/Y

where b indicates single block index and r indicates range.

[chappjc]

#35 also a URL query arg to get pool values and sizes for a range of blocks in their own arrays instead of an array of structs.

For example, /api/stake/pool/r/117723/117724?arrays=true gives you this:

{
  "start_height": 117723,
  "end_height": 117724,
  "value": [
    1988282.95249548,
    1988073.46390894
  ],
  "size": [
    41007,
    41002
  ]
}

But without arrays=true, you get:

[
  {
    "size": 41007,
    "value": 1988282.95249548,
    "valavg": 48.48642798
  },
  {
    "size": 41002,
    "value": 1988073.46390894,
    "valavg": 48.48723144
  }
]

[chappjc]

For this to be done:

• A human readable directory
• ?

Please @fai04d and @raedah and @karamble, let me know what else this needs.