Only show Achievements if certain plugins are active

Hey Paul,

Firstly, great work on Achievements. I'm amazed by how much work you've done in 3.0+ :)

I'm currently doing some prototyping for a potential job and thought that it from a UI point of view it would make more sense to only show achievements for a plugin if it's activated.

In my first pass I checked if plugins were active in the "Plugin extensions" section of achievements.php but then I realised that if I did that it broke the Supported Plugins section of Achievements.

This is my second pass which fixes that problem by tackling it in a different way. It looks like there is some whitespace issues in the patch...I blame PhpStorm & Windows for that :)

Hopefully you can sneak this in 3.3 if you haven't shipped it yet :)

I've attached some screenshots to show my testing as well. Hopefully we'll get this job too and I'll be able to add loads more patches!

![BuddyPress and WordPress are active](https://f.cloud.github.com/assets/1377956/470572/8b716880-b6fc-11e2-8d64-979e7e091b78.png "BuddyPress and WordPress are active")
![Supported Plugins Page](https://f.cloud.github.com/assets/1377956/470573/8b7253f8-b6fc-11e2-8f1b-a20a88f9e8d1.png "Supported Plugins Page")
![Supported Features Page Without bbPress Activated](https://f.cloud.github.com/assets/1377956/470574/8ba81010-b6fc-11e2-9459-a94ff14a31af.png "Supported Features Page Without bbPress Activated")
![Supported Features Page When bbPress Isn't Installed](https://f.cloud.github.com/assets/1377956/470575/8ba8ff16-b6fc-11e2-8215-abbe77d8e37f.png "Supported Features Page When bbPress Isn't Installed")

I hope you're having fun working in VIP too :+1: 

Cheers!