Fixed server list map name casing issue

Determining whether or not a user has a map downloaded depended on the casing of the map name. This is an issue when the casing for a map name is different even though they may have it downloaded.

Here's an image of the issue fixed
![fixed](http://samuelmaddock.com/up/2013-06-29_17-10-00.png)

Since `MapIndex` is only used for checking if the user has a map, the change doesn't affect any other features. I also changed it to simply use a boolean since it would likely be wasting memory otherwise.