[SPARK-1100] prevent Spark from overwriting directory silently 

Thanks for Diana Carroll to report this issue (https://spark-project.atlassian.net/browse/SPARK-1100)

the current saveAsTextFile/SequenceFile will overwrite the output directory silently if the directory already exists, this behaviour is not desirable because

overwriting the data silently is not user-friendly

if the partition number of two writing operation changed, then the output directory will contain the results generated by two runnings

My fix includes:

add some new APIs with a flag for users to define whether he/she wants to overwrite the directory:
if the flag is set to true, then the output directory is deleted first and then written into the new data to prevent the output directory contains results from multiple rounds of running;

if the flag is set to false, Spark will throw an exception if the output directory already exists

changed JavaAPI part

default behaviour is overwriting

Two questions

should we deprecate the old APIs without such a flag?

I noticed that Spark Streaming also called these APIs, I thought we don't need to change the related part in streaming? @tdas