TTTUnitOfInformationFormatter with numbers bigger than 32bit

I found a problem when trying to format a number representing 16GB on an iPhone 4S:
```
NSNumber *sixteenGigsInBits = 137438953472;
TTTUnitOfInformationFormatter *unitOfInformationFormatter = [TTTUnitOfInformationFormatter new];
NSString *sixteenGigsString = [unitOfInformationFormatter stringFromNumberOfBits:sixteenGigsInBits];
```
```
po sixteenGigsString
16,777,216.005 KB
```
After the changes in this commit:
```
po sixteenGigsString
16 GB
```