Add support for form reset functionality.

[Haprog]

Fix issue #111
Tokens are now automatically updated after form reset to reflect the new input value after reset.

[Haprog]

I also made an enhancement to this that triggers a new custom event 'tokenfield:postreset' (as in "pre-/post-") within the timeout function after the value has been (potentially) changed due to form reset. This event is useful if you for example want to run your custom validation code after the value has been changed by form reset.

This is the enhanced implementation:

    // Update tokens on form reset
    this.$element.closest('form').on('reset', function (e) {
      var form = this;
      var oldValue = _self.$element.val();

      // Set a timeout to run code after the reset event is finished
      window.setTimeout(function(){
        var newValue = _self.$element.val();

        // If the value changed due to form reset then update tokens
        if (oldValue != newValue) {
          _self.setTokens(newValue, false, false);
        }

        var eventOptions = {
          attrs: {
            oldValue: oldValue,
            newValue: newValue
          },
          relatedTarget: form
        };
        var postResetEvent = $.Event('tokenfield:postreset', eventOptions);
        _self.$element.trigger(postResetEvent);
      }, 0);
    });

But I didn't publish this yet so if this is ok, I can either push it to my branch for this pull request or make a new pull request after this has been merged unless you want some changes to this.

[Haprog]

I just realized that this event listener should probably be moved to Tokenfield.listen method and unbound in Tokenfield.destroy is this right?

I can make the changes if you confirm that this is how it should be done.

[ragulka]

@Haprog Good catch. This should definitely be in the listen method as you pointed out. Once you change that, I can merge this in. Would you mind taking a stab at writing a test for this as well?