Adding spark.streaming.blockInterval property

This pull request adds spark.streaming.blockInterval property.  Units are in milliseconds and it defaults to 200.

Background info: https://groups.google.com/forum/?fromgroups=#!topic/spark-users/Ow1RiY60oOI

We have used it in our standalone spark streaming cluster today and results have been great.  It has significantly reduced our inode usage and shuffles are much faster now.