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
white list AsIs class. closes #453
test fix
mutate(rowwise_df) makes a rowwise_df. closes #463
commenting out the Rcpp devel branch as this takes too long and Rcpp was released recently so we should not fear problem right now
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
  * Force USE dbname as hive2 driver does not honour URI
  * Fix column alias
  * column alias is not always calculated correctly as it comes
    from a nested sub query
  * fix db_list_tables to scope to DB
  * ImpalaDB can't create temporary tables
  * add option to documentation
Merge remote-tracking branch 'hadley/master'