[SPARK-10836] [SPARKR] Added sort(x, decreasing, col, ... ) method to DataFrame

the sort function can be used as an alternative to arrange(... ).
As arguments it accepts x - dataframe, decreasing - TRUE/FALSE, a list of orderings for columns and the list of columns, represented as string names

for example:
sort(df, TRUE, "col1","col2","col3","col5") # for example, if we want to sort some of the columns in the same order

sort(df, decreasing=TRUE, "col1")
sort(df, decreasing=c(TRUE,FALSE), "col1","col2")

Author: Narine Kokhlikyan <narine.kokhlikyan@gmail.com>

Closes #8920 from NarineK/sparkrsort.