Fix compilation error on Ubuntu 14.04 (gcc > 4.7).
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
Fix typo.
Merge pull request #50 from domob1812/fix-14.04

Fix compilation error on Ubuntu 14.04 (gcc > 4.7).
Merge pull request #53 from domob1812/fix-typo

Fix typo.
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.
Check earlier for blocks with duplicate transactions to prevent DoS.
Port from Bitcoin be8651dde7b59e50e8c443da71c706667803d06d