Bug fixes and added tests

• Bug fixes:

  • In an attempt to keep value property up-to-date, it used to reset its value property with every change to any of its attributes. And when the element is two-way-data-bound using for example Polymer, these updates were reflected to the model (as in <juicy-select options="{{model.Countries}}" value="{{model.value}}">). This caused updates to have side-effects, especially in an async environment. If you change model.Countries in any way, model.value would be updated unintenionally. Potentially causing data-loss or keeping outdated data after it's due.
  • It didn't remove change event listener from the native <select> element inside it upon detachment. Now it does.

• Added tests: Now the element has some tests that definetly can be expanded. But are enough to enable CI.