allow data frame columns again
`rbind`ing a column full of `NA` is allowed and just collects the appropriate
  missing value for the column type being collected. closes #493.
Internal impl of `nth` falls back to R evaluation when `n` is not a length one
  integer or numeric, e.g. when it is an R expression to evaluate. closes #734.
`row_number`, `min_rank`, `percent_rank`, `dense_rank`, `ntile` and `cume_dist`
  handles data frames with 0 rows. closes #762.
lift protection against data frame as columns in LazySubsets
Added the internal class `DataFrameColumnVisitor` to represent `VectorVisitor`
  concept for a column that is a `data.frame`. Part of #602.
Added the internal class template `MatrixColumnVisitor` to represent
  `VectorVisitor` concept for a column that is a `matrix`. Part of #602
merge changes from master branch
`arrange` can sort based on `data.frame` columns (#765)
`arrange` can sort based on `matrix` columns (#765).