Log all changes of user trust level by an admin

Following on from https://github.com/discourse/discourse/pull/1131 and @nlalonde's comment https://github.com/discourse/discourse/pull/1122#issuecomment-20369704 

Also see [discussion of this feature on meta](http://meta.discourse.org/t/feature-mod-manually-sets-a-users-trust-level/8021)