fix(ngSwitch): don't leak when destroyed while not attached

The leak can occur when ngSwich is used inside ngRepeat or any other
directive which is destroyed while its transcluded content (which
includes ngSwitch) is not attached to the DOM.

Refactor ngSwitch to use controller instead of storing data on compile
node. This means that we don't need to clean up the jq data cache.
Controller reference is released when the linking fn is released.

Closes #1621