Rename Sleep to MilliSleep and always use boost.

Ported from phelix' patch in Namecoin.  See:
  https://github.com/namecoin/namecoin/pull/106
  https://github.com/namecoin/namecoin/pull/107

Commits in Namecoin:
  c99ecc9035d00dd1187a898593b3eb0618f7a54d
  bde135cc22399984a32712c39fde9cc7c9772f1d
Add difficulty field to getblock / getblockbycount.

Add the difficulty as new field in the JSON representation of a block.
See https://github.com/namecoin/namecoin/issues/112.
Merge pull request #51 from domob1812/millisleep

Rename Sleep to MilliSleep and always use boost.
Merge pull request #54 from domob1812/add-difficulty

Add difficulty field to getblock / getblockbycount.