More efficiently update display tables

Always shorten rule by 1 character relative to window width

With Emacs 24.4, this seems to yield a full-length rule without a gap at
right. On older versions, it might cause the rule to be too short. It
might be necessary to include an Emacs version check for optimal
behaviour.

Also shorten rule by one character in console windows (fixes purcell#6)

Rule can apparently extend safely to the full window width

Merge pull request purcell#2 from drothlis/derived-modes

Enable in derived modes of `page-break-lines-modes'

Indentation fix

Rename face from page-break-lines-face to page-break-lines

Convert to separate minor-mode and global-minor-mode variants.

Add a basic README

Initial commit