Squashed 'packages/mr/' changes from dd134b1..96d6276


git-subtree-dir: packages/mr
git-subtree-split: 96d6276866b274e7cc35ae208941bf384a169998
Update Mr to v0.15.2

Conflicts:
	packages/mr/browser.js
Merge pull request #1423 from 3on/overlay-typo

Fix a doc typo for willPositionOverlay delegate
Merge pull request #1421 from Stuk/mr0.15.2

Mr v0.15.2
Fix bug induce by an object removing itself from the array of listeners.