- With label2call and labels2expression, should dummies support other types of 
  dummy variables besides categorically encoded, e.g. continuous with margins?

- write testthat tests 

x attributes( data, 'dummies')
  We currently store all the indices, since they are always continuous we 
  need to only store the begining and the end.  Small price to pay,  Refused.

x eliminate which.dummies and get.dummies? No. This is useful.

- When subsetting a dummy frame it would be nice to track the dummy attributes
  - I am not sure how this can be accomplished as attributes usually don't 
    travel with the subset
  - Use S4?

- Add dummy.sep option e.g. options(dummy.sep)

- The exception for variables of one level is problematic see dummy.R
