[C] Fixed setTexturePack to work with Resource Packs. Fixes BUKKIT-5015

**The Issue:** The setTexturePack is not working correctly in Minecraft 1.6+ due to the new Resource Packs. 

**Justification for this PR:** The setTexturePack can work again in Minecraft 1.7 with the new Server Resources.

**PR Breakdown:** Changed setTexturePack CustomPayload channel from "MC|TPack" to "MC|RPack".

**Testing Results and Materials**
Use /test `resourcepack url`
Binary: http://www.sendspace.com/file/nr032k - Source: http://pastebin.com/uQFNmJpH
Screenshot: http://img69.imageshack.us/img69/6576/l2hg.png

**JIRA Ticket:** BUKKIT-5015 - https://bukkit.atlassian.net/browse/BUKKIT-5015