Add support for file request paging to Remote::FindParentById

While issuing pull requests on a few of my directories I noticed that
I was missing files. Implement paging so pull requests on directories
that have more than 100 files work.
ID should have been typed as `ID` and named return values are actually should be avoided. While you're there, could you modify the signature to be the following?

func (r *Remote) FindByParentID(parentID string) ([]*File, error) {