SPARK-1637: Clean up examples for 1.0

1. Move all of them into subpackages of org.apache.spark.examples (right now some are in org.apache.spark.streaming.examples, for instance, and others are in org.apache.spark.examples.mllib)
2. Move Python examples into examples/src/main/python
3. Update docs to reflect these changes
4. Clarify that the hand-written K-means and logistic regression examples are for demo purposes, but in reality you might want to use MLlib (we will add examples for these using MLlib too)