Implements PoW validator

Implements: #35 (closed)

lib/block/validation/local/proof_of_work.ex

+defmodule Block.Validation.Local.ProofOfWork do
+  def valid(block) do
+    required_zeros = div(block["powMin"], 16)
+    String.match?(block["hash"], ~r/^0{#{required_zeros}}/)
+  end
+end
+