[conformal/btcchain] 190ef7: Export CalcWork and BlocksPerRetarget.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcchain
  Commit: 190ef70aced3bb1892060f72e5fd1b99b6d461d5
      
https://github.com/conformal/btcchain/commit/190ef70aced3bb1892060f72e5fd1b99b6d461d5
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-02-07 (Fri, 07 Feb 2014)

  Changed paths:
    M chain.go
    M difficulty.go

  Log Message:
  -----------
  Export CalcWork and BlocksPerRetarget.

This commit makes the CalcWork function and BlocksPerRetarget variable
available to external packages.