Fix unclosed <a> tag on front page

Closes #1265
[show-hint addon] Make hint window behave halfway proper when scrolling the editor

Closes #1264
Fix selection drawing in non-left-aligned text
Add a getAllMarks method
Make marker widgets inline blocks
Add null/plain text mode to mode/meta.js

Closes #1267
Add beforeSelectionChange event

Issue #1262
[htmlmixed mode] Make detection of opening script/style tags more robust

Closes #1271
[show-hint addon] Close hinting popup when scrolled out of view
[lint addon] Add an option run an asynchronous linter
Add a kludge to work around poor dimension measurements in bidi text in IE

Closes #1129
Clean up documentation for addons, mention lint addon
Mark release 3.1
Bump version post-3.1