dynamic grouping/tree fix

[jonshaffer]

Fix for #212

1. In demo, sets group:true to 1 column (the code only accounts for this anyways at this time)
2. pulled refreshTree, refreshGroup logic into its own method for calling on groupColumn || treeColumn change
3. added a watch on the options.columns that updates group/tree for the table
4. moved conditional scope.watches from the constructor to a method that gets reevaluated if columns ever change