Allow players to track both herbs and minerals at the same time

PR as [requested](http://www.trinitycore.org/f/topic/9390-patch-allow-track-herbs-and-minerals-at-same-time-updated-22-11-2013/#entry68082)

__Note:__

For documentation sake, [here](http://www.trinitycore.org/f/topic/9390-patch-allow-track-herbs-and-minerals-at-same-time-updated-22-11-2013/#entry61754) is a useful macro to allow players to toggle both trackers on / off. Just open the spoiler in the post.
I do not know if it actually happens but this return might be too early, because it will return if the config is enabled or not.

```cpp
if (sConfigMgr->GetBoolDefault("AllowTrackBothResources", false) && m_spellInfo->HasAura(SPELL_AURA_TRACK_RESOURCES) && existingSpellInfo->HasAura(SPELL_AURA_TRACK_RESOURCES))
    return true;
```