[SPARK-2010] [PySpark] support nested structure in SchemaRDD.

add API: applySchema(rdd, schema, f=None)

It need the new API to support nested structure.

It will do inferSchema() in Python, then convert the namedtuple() or other objects into 
tuple() (using dict currently). Before call SchemaRDD.mapPartitions(), It will convert 
the tuple into namdtuple, so user will access name fields like normal objects.

Because the type of returned data from collect has changed, so unit tests still failing, 
will be fixed later.
If you want to add this library in the repo, you need to mention its provenance in LICENSE at the top of the project. Take a look at the entry for CloudPickle there.

Also, is this a replacement for CloudPickle overall? I see a bunch of stuff in it for functions.