Add type='button' to tags with multi-select feature

[jgoux]

Hi,
with the multi-select feature, tags are <button>.
As buttons, they are conflicting with the ng-submit directive if the ui-select is declared in a form. The form is submitted as soon as the user clicks on a tag.
To avoid that, type="button" needs to be added on each tags.

[dimirc]

Thanks for report, I just committed fc42fdf to fix it