Add 'drop' support for DataFrame's subset function

SparkR DataFrame can be subset to get one or more columns of the dataset. The current '[' implementation does not support 'drop' when is asked for just one column. This is not consistent with the R syntax:
x[i, j, ... , drop = TRUE]

1. in R, when drop is FALSE, remain as data.frame
   > class(iris[, "Sepal.Width", drop=F])
   [1] "data.frame"
2. when drop is TRUE (default), drop to be a vector
   > class(iris[, "Sepal.Width", drop=T])
   [1] "numeric"
   > class(iris[,"Sepal.Width"])
   [1] "numeric"

> df <- createDataFrame(sqlContext, iris)

1. in SparkR, 'drop' argument has no impact
   > class(df[,"Sepal_Width", drop=F])
   [1] "DataFrame"
   attr(,"package")
   [1] "SparkR"
2. should have dropped to be a Column class instead
   > class(df[,"Sepal_Width", drop=T])
   [1] "DataFrame"
   attr(,"package")
   [1] "SparkR"
   > class(df[,"Sepal_Width"])
   [1] "DataFrame"
   attr(,"package")
   [1] "SparkR"

We should add the 'drop' support.