Eighth CRAN Release

This update has added the following new features:

• group_by() can now create new columns, grouping by the result (#58).
• with_groups() (#61).
• nth() (#62).
• mutate() can now relocate resulting mutations (#59).
• mutate() can now keep all, used, unused or none of the resulting mutations (#60).
• across() can now accept lambda style functions (#52).
• Columns created by summarise() are now immediately available (#20).
• cur_column() (#52).

The following fixes were also implemented:

• across() can now accept function names as characters (#56) - thanks @eyayaw for pointing this out.
• An empty summarise() will now correctly return an empty data.frame (#55).

Other general refactorisations and improvements of note:

• group_by() is now an S3 generic.
• .default() methods are now .data.frame() methods.
• get_groups() has been removed in favour of group_vars().
• set_groups() has been replaced with groups_set() for API consistency.