dplyr support for cor, cov, sd and var to fix #426
dplyr support for cor, cov, sd and var to fix #426

kevinykuo/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr,rstudio/sparklyr,kevinykuo/sparklyr,rstudio/sparklyr