Added changes in Gen VI to the moves changelog

I've added changes made in Gen VI to the moves changelog. (Issue #127)

While I was going through that, I noticed that it doesn't currently track changes in target and priority, so I added fields for those to the dataset as well (hence why the changes show everything as being changed - two columns were added to the CSV).

The only issue I ran into was that Hidden Power (id 237) had NULL power until Gen VI, where it now has a defined 60 power.  Because NULL in the changelog means that value hasn't changed, I put in 1 for power.