prevent users from liking a post more than once

Part of my morning routine is to check what's new on [meta](http://meta.discourse.org/).
What I usually do is to middle-click every new/unread topics and then check every tab.

This morning, I opened [a topic](http://meta.discourse.org/t/should-you-be-editing-css-manually-or-using-site-customization/6350) twice and for *some reason* I liked the first post twice (in different tabs), resulting in:

![image](https://f.cloud.github.com/assets/362783/458297/c7663e52-b3d1-11e2-8a89-fe76b181bd7b.png)

This pull request prevents this.