Make dropdown openable

[jspdown]

I wasn't able to use the dropdown because of missing classes on <div class="menu">.
This div must have the following classes to work:

<div class="menu transition visible"> </div>

The transition class is not compulsory but add a nice effect
After those change done, I got other issues:

• The scope.open property was supposed to be string but we stored a boolean. So, an equality check with a boolean value failed
• Sometime, the scope.model was updated with the original title and not undefined
• If scope.model == undefined, the title of the dropdown was set to the undefined instead of the original name
• The var title in the dropdown-group wasn't used. So, if no title was provided for this directive, after selection, we set the dropdown title to undefined

Those points are fixed.
Dropdown closed:

Dropdown opened:

This pull-request also include an update of the dropdown/doc/ files

[0xAX]

Thank you @jspdown