Update databases.Rmd

Fixed typo
Merge pull request #431 from nachti/patch-1

Update databases.Rmd
Upgrade to roxygen2 4.0.1
Update submission notes
fix typo in R version number
Merge pull request #432 from paulstaab/master

fix a typo
Bump version
Merge branch 'master' of github.com:hadley/dplyr
Fix typo
typo
Merge pull request #444 from tverbeke/master

typo
move some functions to headers so that we can use dplyr header files standalone. closes #443
fix OrderVisitors( DataFrame data, CharacterVector names ) ctor
Update group-by.r

update group_by documentation to properly reflect change to add = FALSE being the default
Merge pull request #451 from dpastoor/patch-1

Update group-by.r
Update databases.Rmd
handle edge case n_distinct( <0 length vector>). closes #458
mutate( name = NULL ) syntax. #462
handle NA in cumany and cumall. #408
additional test for cumany/cumall