[PR] Changes to the timeline item entry form

[jeremyfelt]

• [x] Modify placeholder text for timeline item post title to be more appropriate. Fixes #4
• [x] Move item submission information to its own meta box. Fixes #6
• [x] Use timeline item instead of timeline point. See #4
• [x] Rename "Story Source Notes" to be more friendly. Fixes #7
• [x] Add title and explanation for description.