Enhancements for *dply

[krlmlr]

• Generate .id column as factor in ldply(); fixes ldply and the like should create the .id column as factor, preserving the order of the names in the list #137
• New parameter .id for rdply() to specify name of index column (default:
  .n); fixes Allow renaming of .n column generated by rdply #142
• New parameter .id for ldply() to specify name of index column (default:
  .id); fixes Allow renaming of .id column generated by ldply #140; fixes plyr does not preserve names in nested calls #107

Fixes #174 (=includes it)

[hadley]

Could you please rebase/merge, update docs and resubmit?