Fixed memory leak

After profiling your code, I saw that you have a memory leak in your FBFontSymbol class at NSMutableArray *symbols = @[].mutableCopy;.  I changed your code to have the NSMutableArray be a singleton so that the array does not get created every time the FBFontSymbol gets used.  That seemed to have solved the issue without tampering too much with your code.  Please take a look at my git pull request and see if this makes sense when you run your memory leak profiler.

![screen shot 2014-02-11 at 9 39 47 pm](https://f.cloud.github.com/assets/38386/2145470/0f460f86-93a6-11e3-947e-50797b7b77bc.png)