separate MCR/MCA branches in region file prep
instructions for first-time maven users
fix: now iterates sections/blocks in anvil files (only)

  still a few outstanding issues. block IDs above 255 are not correctly matched
  because the "Add" tag in each Section is not considered correctly.

  mtimes are modified poorly, causing overviewer to complain when re-mapping the
  edited world. however, the server seems to have no problem reloading the level
  data, and in my first test it also regenerated the spawn area without
  difficulty.

  the code was rewritten without considering backwards compatibility.
  two versions of markSafeChunks() need to be used based on whether the parser
  is handling an MCA or MCR file.