[conformal/btcchain] be2e4c: Add new function to get the checkpoints.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcchain
  Commit: be2e4c5860bdbd2db42c473bb6c31c1954d14022
      
https://github.com/conformal/btcchain/commit/be2e4c5860bdbd2db42c473bb6c31c1954d14022
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-01-31 (Fri, 31 Jan 2014)

  Changed paths:
    M checkpoints.go

  Log Message:
  -----------
  Add new function to get the checkpoints.

Previously there was only a function to get the latest checkpoint.  This
commit exposes a new function named Checkpoints which returns a slice of
checkpoints ordered by height for the active network or nil when
checkpoints are disabled.