Wrap missing `Bindings.create*Binding`

[jpsacha]

create*Binding methods allow creation binding of arbitrary expression created from any number of observable values.

Here is a possible wrapper for createStringBinding:

  def createStringBinding(op: () => String, values: Observable*): StringBinding = {

    jfxbb.Bindings.createStringBinding(
      new jfxuc.Callable[String] {
        override def call() = op()
      },
      values.map(_.delegate): _*)
  }

it allows writing code like, that creates binding that converts String property to lower case:

  val a = new StringProperty()
  val b = createStringBinding(() => Option(a.value).getOrElse("").toLowerCase(), a)

See also discussion at #231.