[btcsuite/btcd] a6c79c: Implement getblockheader RPC.

[David Hill]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcd
  Commit: a6c79c7a913b1f9f5e0a7c2aa9c6416ecd430c84
      
https://github.com/btcsuite/btcd/commit/a6c79c7a913b1f9f5e0a7c2aa9c6416ecd430c84
  Author: David Hill <dh...@mindcry.org>
  Date:   2015-07-23 (Thu, 23 Jul 2015)

  Changed paths:
    M btcjson/chainsvrcmds.go
    M btcjson/chainsvrcmds_test.go
    M btcjson/chainsvrresults.go
    M docs/json_rpc_api.md
    M rpcserver.go
    M rpcserverhelp.go

  Log Message:
  -----------
  Implement getblockheader RPC.

This mimics Bitcoin Core commit 076badb60f33f0c32b035de220ca14c52a423a2a