refClass implementation from Ryan Thompson
preliminary refClass revisions

- precede fields not intended for exposure to users with '.'
- $show rather than show,...
- $initFields in initialize methods
- need to initialize MulticoreParam to satisfy validity of new()
- only validate fields of class, not of parents
- separate zzz.R into shared functionality (utilities.R) and
  platform-specific, e.g., unix/zzz.R
- misc formatting changes
use setLoadActions for .onLoad
initialize BiocParallelParam with workers arg
tidy SnowParam-class