Various enhancements and fixes for .parallel = T

- Use a wrapper to call .fun in llply; this avoids warnings when using `.parallel=T` with a snow backend. (Fixes #203)

- error messages are also reported with .parallel = TRUE

- `do.ply` does not check `.inform`, instead the code chooses between two
  slightly different versions of `do.ply`.  This should result in marginally
  faster execution