Make SUStandardVersionComparator.h accessible

This pull request is related to [#11](https://github.com/pornel/Sparkle/issues/11)

The standard version strings comparator is very useful for basic comparisons, but inaccessible once a custom object is set as the comparator.  
There are two changes, the first puts `SUStandardVersionComparator.h` in the public header group, the second includes it by default in `Sparkle.h`.