Fourth CRAN Release

This update has added the following new features:

• if_else() (#11)
• between() (#12)
• lead() and lag() (#13)
• n_distinct() (#16)
• window rank functions: cume_dist(), dense_rank(), min_rank(), ntile(), percent_rank(), row_number() (#18)

My thanks go to @msberends for their help on these features!

This update also refactors a lot of the code, hence the minor version bump. This includes:

• More support for "tidyselect" features such as !, :, -, c() and ()
• Removal of eval(parse(text = ...)
• Rework of grouped summaries

Finally, 0.2.0 includes many bug fixes:

• Joins now maintain their original order (#10)
• group_by() + mutate() now preserve row order (#14)
• Quote names are allowed in pull() (#15)