update Issue Watchers - requires Redmine v. 2.3+

[alexeyOnGitHub]

there's no way to add/remove followers for issues.

Alexey: REST API does not support this. http://www.redmine.org/issues/13029

[alexeyOnGitHub]

this is added to Redmine trunk branch. it will be a part of their 2.3 release. see http://www.redmine.org/issues/6727