For issue 32 & 29

Implements: #29 (closed) Implements: #32 (closed)

For issue 32(local validation of previousIssuer) & 29(local validation of UnitBase)

lib/block/validation/local/check_previous_issuer.ex

+defmodule Block.Validation.Local.PreIssuer do
+  def valid(block) do
+    not (block["number"] == 0 and block["previousIssuer"] != nil) and not (block["number"] > 0 and block["previousIssuer"] == nil)
+  end
+end