fixes #81, #104

[morficus]

adding new attrForSelected attribute to specify an attribute whose value to use as this elements value

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[zmoshansky]

@morficus
Would you consider the following: value = selectedItem[this.contentElement.attrForSelected] || label; ?

The reason being that attr-for-selected stays on the paper-menu/paper-listbox contained in the content section. This is useful since paper-dropdown-menu doesn't have any way of setting the selected item. Selection must be done on the paper-listbox, which means that one may have to duplicate the attr-for-selected on the paper-listbox.

It would enable the following:
eg.)

let _bar = 0;
....
<paper-dropdown-menu>
  <paper-listbox selected="{{_bar}}" attr-for-selected="foo" class="dropdown-content">
    <paper-item foo="0">Zero</paper-item>
    <paper-item foo="10">Ten</paper-item>
    <paper-item foo="20">Twenty</paper-item>
  </paper-listbox>
</paper-dropdown-menu>

[morficus]

This is useful since paper-dropdown-menu doesn't have any way of setting the selected item. Selection must be done on the paper-listbox

Very good point about the selection staing with in the list-element it self 👍 I'll make the change and update the PR

[zmoshansky]

@cdata Could you please review?

[cdata]

I have left some feedback on this design on #104