IDBKeyRange has static methods that should be defined in scala object

[IDBKeyRange](http://www.w3.org/TR/IndexedDB/#idl-def-IDBKeyRange) should be like this right?

But currently it is just a class [IDBKeyRange](https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/Idb.scala#L355) and if you call 
```scala
new IDBKeyRange().bound("a","z")
```
you'll get `TypeError: IDBKeyRangeConstructor is not a constructor (evaluating 'new ScalaJS.g["IDBKeyRange"]()')`

I tested the solution I suggested, doing : 
```scala
IDBKeyRange.bound("a","z")
```
resolves to `IDBKeyRange` instance