Move buttons to sx-button.el

Also define command to copy button's content under w.
Fix button-map
Improve button-copy
Normalize and improve button help-echo's
New sx--shorten-url function in sx.el
Better help-echo in links
New "Add a Comment" button
Slight tweak to question-mode-recenter-line

So the "add comment" button is visible after navigating to the comments
section.
Woops. Fix swallowing one function inside the other.
Add sx--unindent-text function

Used to extract the code contained in code-blocks
Code-blocks are buttons. RET to edit, w to copy.
Comments
Remove bad require
Add needed require
Clean 'display in code-blocks. Fixes #117
question-mod now uses overlays to store the sx--data-here property

This lets us stack one inside each other (comments inside questions)
without overwriting them.
Better positioning of the add comment button
Comments are now hidden when you fold the question
Switch get-pos-property to get-char-property
Merge pull request #124 from vermiculus/buttons-everywhere

Buttons everywhere