Simpler `execCommand` logic
Argument validation in _exec
Fix Trident 7's docEventQuirks
Fix some image related tests
Remove old image selection test
Merge pull request #651 from wymeditor/qunit-hide-passed

QUnit: Hide passed tests
`_exec`: better error messages
Merge pull request #650 from wymeditor/_exec

Some `_exec` optimizations
Merge branch 'master' into selectImagesWithRangy
Don't use sinon fake timers in QUnit.
manipulationTestHelper: async feature
Async image selection test for pre-7 Trident
Fix some image related tests for pre-7 Trident