added methods to add and remove labels with an Object tag

[josephusvillarey]

I've encountered some cases during the development of my app where it is not enough to add a label using a String or index tag, like when labels can be of the same text representation but represent different object in a selection, or when you're trying to remove a label that corresponds to a certain checked item in your list, but the text on the label is not unique. This pull request solves that issue by being able to add a label with an Object as a tag. This can be useful when you want to pass your own model object as a tag; just make sure that your object properly overrides the equals() method so the removeLabel(Object tag) method can properly remove the corresponding label with the given tag.