Return data as a tibble

[bryceroney]

This is a bit of a stylistic preference but my understanding is that tibbles are now preferred to data.frames.

This PR:

• Adds tibble as a dependency
• Uses tibble::tibble for returned data
• Renames {to_df} functions to {to_tibble}
• Supplementary updates to documentation from runing devtools::document()

No dramas if you prefer data frames though

[gaborcsardi]

Thanks! Strictly speaking, this is a breaking change, because tibbles behave slightly different than data frames, so I think I will not merge this now.

It is easy to call tibble::as_tibble() on the result.

If we started today, then we would probably use tibbles.

[bryceroney]

Fair enough.