Better auto-detection of paths

Having installed MySQL using a recent community installer, I found the RMySQL path detection to be insufficient. This code here seems to work for me.

If you don't agree with all my commits, feel free to merge only some of them. If you want an updated pull request for that, please let me know.

Note that I mentioned issues with the 64-bit registry access back in 2011, but apparently you didn't merge my patches back then. Which might be a good thing, because the 32-bit view is much more elegant than explicitely naming the Wow6432Node in the registry path.