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
src-impaladb: first commit of ImpalaDB support

  * includes modifications to support explict column scoping
    required to support joins with ImpalaDB
  * semi-joins are done through the explicit LEFT SEMI JOIN syntax
  * loading tables requires HDFS support via rhdfs
  * note: IMpapaDB does not support anti-joins