Parallel chunk.apply support.

1. chunk.apply now dispatches to either the seq.chunk.apply or mc.chunk.apply function. Both are private.
2. There is a new "parallel" parameter for chunk.apply. The default is 1. If more than 1 is specified an you are on a unix machine the chunk.apply occurs in parallel and chunks are pre-fetched.
3. The documentation has been updated to reflect the new parameter.
4. The minor version has been bumped.