Remove confusing message on mode activation

The call to 'toggle-save-place' evokes the message
'No place will be saved in this file', which is just annoying
and confusing.

Directly setting 'save-place' is safe – it's part of saveplace.el's API.
Fix highlighting of non-empty second line

The last group must be capturing. Its value is used later on for
highlighting.
Plus: Match second lines that have only one char.
Fix highlighting of summary lines without trailing newlines

This restores instant highlighting while the user is entering the summary
line.
Fix matching of empty lines before summary

The old regexp didn't match lines that were completely empty.
Merge pull request #49 from nonsequitur/fixes

Fixes