Added GetIndices function to TreePath type.

Hi,

I added a Go wrapper for the [gtk_tree_path_get_indices](http://www.gtk.org/api/2.6/gtk/GtkTreeModel.html#gtk-tree-path-get-indices) function.

This code converts a C array of `gint` to a Go `[]int` slice. I'm not quite sure, if this is the correct way of reading/converting a C array with cgo, but it seems to work fine. If there is a better way, please let me know, and I'll implement it.