Handler[T,R] should be newable/extendable in newer Scala.js

[exoego]

Closes #83

A Scala.js-defined JS object cannot directly extend a native JS trait in:

• Scala.js 0.6 with -P:scalajs:sjsDefinedByDefault option
• Scala.js 1.0

Currently, Handler extends js.Function3, so users in such setup cannot extend Handler or construct (new Handler {}).

This PR changes Handler[TEvent, TResult] into a Scala.js-defined trait.
Note that Handler and AsyncHandler is now clearly distinguished.

trait Handler[TEvent, TResult] extends js.Object {
  @JSName("apply")
  def apply(event: TEvent, context: Context, callback: Callback[TResult]): Unit
}
trait AsyncHandler[TEvent, TResult] extends js.Object {
  @JSName("apply")
  def apply(event: TEvent, context: Context): js.Promise[TResult]
}

so that it is newable/extendable

import scala.scalajs.js
import net.exoego.facade.aws_lambda._

object MyFirstLambda extends APIGatewayProxyHandler {
  @js.annotation.JSName("apply")
  override def apply(event: APIGatewayEvent, context: Context, callback: Callback[ProxyResult]): Unit = ???
}
object MyFirstAsyncLambda extends AsyncAPIGatewayProxyHandler {
  @js.annotation.JSName("apply")
  override def apply(event: APIGatewayEvent, context: Context): js.Promise[APIGatewayProxyResult]  = ???
}