Allow chaining sibling's scheduled time.

Allow chaining sibling's scheduled time.

Patch by Andrew Hyatt

Andrew Hyatt writes:

    I like to schedule items I'm supposed to be working on, so I like
    when I complete a task that the next sibling task gets the
    schedule from the previous one.  This implements that kind of
    workflow with the org-depend contrib package.

    I've also added a few methods to org.el to make working with
    schedules & deadlines easier.