Parallel processing of streamed input, tests, and other misc improvements

• Bug fixes to maxzoom, and more tests
• There are tests that can be run with "make test".
• Feature properties that are arrays or hashes get stringified
  rather than being left out with a warning.
• Make clipping behavior with no buffer consistent with Mapnik.
  Features that are exactly on a tile boundary appear in both tiles.
• Parallel processing of input with -P works with streamed input too
• Error handling if unsupported options given to -p or -a