Support for Async requests and custom item unique ID

[dfabreguette]

Suggests 2 things :

• Changed userDidSearch method to pass tableView :

func userDidSearch(searchString:String, tableView: UITableView)

It allows to reload tableView once my async request has given me back remote data.

• Added a string unique id (provided by user through SwiftMultiSelectItem constructor).
  It allows to make sure if we do multiple async searches that changes the tableView data, we base the selection condition on this unique ID and not the tableView row index.