Implement `reframe()`

[domq]

This is a successor to #102 of sorts.

• What I am trying to do:

  library(dplyr)
  library(multidplyr)
  # install.packages("broom")
  
  cluster <- new_cluster(2)
  iris %>% group_by(Species) %>% partition(cluster) %>% reframe(broom::tidy(lm(Petal.Length ~ Sepal.Length)))
  

• What I expect: the same data frame that I obtain when doing that without the %>% partition(cluster) part (modulo performance).
• What I get instead:

  Error in UseMethod("reframe") :
  no applicable method for 'reframe' applied to an object of class "multidplyr_party_df"
  

[hadley]

Thanks for the suggestion! Will definitely consider it when I'm next working on multidplyr.