Second CRAN release

Second CRAN release.

This update adds selection helper functions. See ?select_helpers for more information.

The following bugs have been fixed:

• mutate() column creations are immediately available, e.g. mtcars %>% mutate(mpg2 = mpg * 2, mpg4 = mpg2 * 2) will create columns named mpg2 and mpg4
• group_by() groups now persist in selections, e.g. mtcars %>% group_by(am) %>% select(mpg) will return am and mpg columns
• slice() now duplicates rows, e.g. mtcars %>% slice(2, 2, 2) will return row 2 three times
• summarize() is now exported