Update link to Grunt plugin in phantomjs addon.

Closes #430.
Gruntfile: Add watch task.
Enforce stricter jshint settings.

Core:
- Deprecate QUnit.current_testEnvironment.
- extend() no longer copies over prototype.
  Since QUnit is an instance of itself (a hack to differentiate
  between QUnit[event]=fn and QUnit.event(fn)), fix usages
  of extend(.., QUnit) to QUnit.constructor.prototype.

Test:
- Variable fn2 was unused. Added a test for it (diff6).
- Variable diff/same were unused, converted into a comment.
Deprecate QUnit.current_testEnvironment in favour of config.current.testEnvironment.
Clear filter and testNumber when choosing a module. Fixes #442.
License: Bump to 2013 and resolve redirect license url
Add MIT-LICENSE.txt
Add two more missing authors
Add last missing author. Fixes #413
Canvas plugin: Show how to test with images. Closes gh-438.
canvas addon: Fix jshint warnings

Follows-up 608298208b0feb5a235101
addons: Phase out pattern using global functions
Remove bad comment, leftover from rebasing