[SPARK-3841] [mllib] Pretty-print params for ML examples

Provide a parent class for the Params case classes used in many MLlib examples, where the parent class pretty-prints the case class fields:
Param1Name	Param1Value
Param2Name	Param2Value
...
Using this class will make it easier to print test settings to logs.

Also, updated DecisionTreeRunner to print a little more info.

CC: @mengxr
This may not look pretty in the WebUI:

![screen shot 2014-10-07 at 5 36 36 pm](https://cloud.githubusercontent.com/assets/829644/4552499/5d241b2c-4e85-11e4-8934-0c162038429b.png)

We can output in a JSON-like format:

~~~
{
  maxIter: 10,
  regParm: 1.0
}
~~~