Implement dapply() on DataFrame in SparkR

dapply() applies an R function on each partition of a DataFrame and returns a new DataFrame.

The function signature is:

	dapply(df, function(localDF) {}, schema = NULL)

R function input: local data.frame from the partition on local node
R function output: local data.frame

Schema specifies the Row format of the resulting DataFrame. It must match the R function's output.

If schema is not specified, each partition of the result DataFrame will be serialized in R into a single byte array. Such resulting DataFrame can be processed by successive calls to dapply() or collect(), but can't be processed by normal DataFrame operations.