Follow Microsoft's naming convention

I've created a branch where I've fixed the naming convention of `SharpRaven.Data.ErrorLevel`. I've also added documentation to the enum so it's easier to pick the right level and correlate the level with the errors that show up in the stream (which level has which color, etc). If I find other areas in the code where [Microsoft's Design Guidelines](http://msdn.microsoft.com/en-us/library/ms229042.aspx) aren't followed, I'll push them to the same branch.