Made the README nicer, and some other small tweaks

Updated the README (syntax highlighting, more examples, more descriptive); see here: https://github.com/epitron/coolline/blob/master/README.md
Added Coolline.readline (for Readline.readline compatability)
Added Coolline#bind (for binding keys on the Coolline instance)
Fixed a bug (when passing default_line to #readline, default_line wasn't dup'ed, which could clobber the caller's string)

Mon-Ouie / coolline