Basic scroll past end support
Consider content height when calculating past-end
Add preference option to toggle scrolling past end
Ignore trailing junk when scrolling past end

To avoid performance hit, content size update is noew performed
only when scrolling past end is enabled. Some refactoring are
done to reuse parts of existing calculation.
Make scrollsPastEnd property atomic
Merge branch 'scroll-past-end'

Fix #60

Conflicts:
	MacDown/Code/Document/MPDocument.m
	MacDown/Localization/Base.lproj/MPEditorPreferencesViewController.xib
        (resolved with "theirs")