added support for data-cache=false on modals

when loading a remote model via a link the response is cached in the DOM
sometimes we want to load the remote each time, this allows you to add
data-cache=false to the link

e.g.
<a href="/create" data-toggle="modal" data-target="#modal"
data-cache="false">Create</a>

when the model is hidden the component is destroyed and the cached model
content removed
see  Modal.prototype.hideModal

test have been added to check data-cache and backward compatability