[SPARK-4397][Core] Reorganize 'implicit's to improve the API convenience

This PR moved `implicit`s to `package object` and `companion object` to enable the Scala compiler search them automatically without explicit importing.

It should not break any API. I compiled the following codes with Spark 1.1.0:
```scala
import org.apache.spark.{SparkContext, SparkConf}
import org.apache.spark.SparkContext._

object ImplicitBackforwardCompatibilityApp {
  def main(args: Array[String]): Unit = {
    val conf = new SparkConf().setAppName("ImplicitBackforwardCompatibilityApp")
    val sc = new SparkContext(conf)

    val rdd = sc.parallelize(1 to 100).map(i => (i, i))
    val rdd2 = rdd.groupByKey() // rddToPairRDDFunctions
    val rdd3 = rdd2.sortByKey() // rddToOrderedRDDFunctions
    val s1 = rdd3.map(_._1).stats() // numericRDDToDoubleRDDFunctions
    println(s1)
    val s2 = rdd3.map(_._1.toDouble).stats() // doubleRDDToDoubleRDDFunctions
    println(s2)
    val f = rdd2.countAsync() // rddToAsyncRDDActions
    println(f.get())
    rdd2.map { case (k, v) => (k, v.size) } saveAsSequenceFile("/tmp/test_path") // rddToSequenceFileRDDFunctions

    val a1 = sc.accumulator(123.4) // DoubleAccumulatorParam
    a1.add(1.0)
    println(a1.value)
    val a2 = sc.accumulator(123) // IntAccumulatorParam
    a2.add(3)
    println(a2.value)
    val a3 = sc.accumulator(123L) // LongAccumulatorParam
    a3.add(11L)
    println(a3.value)
    val a4 = sc.accumulator(123F) // FloatAccumulatorParam
    a4.add(1.1F)
    println(a4.value)

    sc.stop()
  }
}
```
And ran it with this PR. It ran correctly.

However, for `WritableConverter`, I cannot make it work without `import`. Thoughts?

The disadvantage is there are two copies of same codes now.
> My guess is that it needs to be public.

If it's `private[spark]`, people are still able to use it implicitly. But they cannot import these methods or call them directly. In other words, my question is should we enable people to import them or call them directly.