Add a must_include_parent option to Trail and Index.

It is useful when your logical paths should need to include the last directory name that was appended to trail paths. E.g.

    t = Trail.new '.', { :must_include_parent => true }
    t.append_paths '/opt/some/project1', '/opt/some/where/else/project2'

    t.find 'project1/foobar.txt'        # Instead of 'foobar.txt'
    t.find 'project2/bla/another.txt'   # Instead of 'bla/another.txt'

Doing it this way is cleaner than adding '/some/' and '/some/where/else/' as the Trail paths, since you'd have to double check that you were not accidentally finding files that were in peer directories of project1 and project2.
Tweaked the read me to talk about the new option.