Add bounds checking to line counting - fixes #3

@aspiers - If you could do a quick review over this that would be great. The change to `smooth-scroll-lines-from-window-top` wasn't strictly necessary for this fix, but I figured it would be good to keep them symmetric. I also bumped the version number to 1.0.3 and updated the Change Log.

*Details*
The 'smooth-scroll-lines-from-window-*' functions were capable of
passing bad range parameters to the 'count-lines' and
'count-screen-lines' functions. Now they will simply return 1 whenever a
bounds check fails.