Stop trying to use event.buttons in recent IEs

It is unreliable, and cleared for no apparent reason if when buttons
are still held.

Issue #2577
Use an ie_version variable, instead of multiple ie_uptoX flags

This makes the version tests more readable (though also more verbose).
Fix copy-pased title in demo/markselection.html
Introduce singleCursorHeightPerLine option

Not official/documented for now.
Clean up singleCursorHeightPerLine implementation
Fix jsbin link in reporting.html
[coffeescript] remove license link
[r] update link to twitter..

..website isn't available
Restore version field in bower.json
Make changeLine take a document, not an editor

Issue #2573
Move add/removeLineClass methods to document

The editor instances will inherit them from there.

Issue #2573
[real-world uses] Add Coderpad
[active-line addon] Don't highlight line(s) with a selection.

This fixes non-empty selections having 2 highlights: the selection highlight
and the active-line highlight. Non-empty selections should only have the
selection highlight.
[vim keymap] visual_gv reimplementation