FIX: @username mentions when there are multiple code blocks in a post.

Meta: [All of my internal users show as coming from 127.0.0.1!](http://meta.discourse.org/t/all-of-my-internal-users-show-as-coming-from-127-0-0-1/6607)

This fixes the regular expression used to identify @ inside <code> or <pre> blocks in the `postConversion` hook.
Merge pull request #851 from ZogStriP/fix-user-mention-when-there-are-multiple-code-blocks

FIX: @username mentions when there are multiple code blocks in a post.