Fix use of exceptions

Assigning to TimesItem.startTime when in a SeqGroup should result in an
InvalidStateError.

TimedItem.endTime is read only, so attempts to write to it should fail silently
and should not throw an exception. See
http://www.w3.org/TR/WebIDL/#es-attributes
Make TimedItem.parentGroup read only

This was agreed at the recent face-to-face meeting.
Remove superfluous setTimeout() from test-seq-speed.html
Style fixes
Adds loadEditor() to load the try-it-yourself object
Add a tutorial for parallel groups
Fix a bunch of style
Merge pull request #90 from steveblock/tutorial

Add the remaining parts of the tutorial framework
Fix style in test-circle-square.html
Fix a bug in TimedItem._iterationTimeToItemTime()

When the current iteration direction is backwards, invert the iteration time
delta. This has the effect of using a start time relative to the end of the
iteration interval for reversed iterations.
Use stable sort when calculating the compositing order

This fixes https://github.com/web-animations/web-animations-js/issues/81
Merge pull request #89 from steveblock/parentNode

Make TimedItem.parentGroup read only
Merge pull request #92 from steveblock/sandyBug

Fixes https://github.com/web-animations/web-animations-js/issues/81
Revert "Remove superfluous setTimeout() from test-seq-speed.html"

This setTimeout() is used to test re-running the animation with a playbackRate
of 2.

This reverts commit b3908add33583c9dbf94d6d0f041621d8c329eba.
Modify test-seq-speed to use a playbackRate of 2 for the first iteration

This avoids the need to wait for the setTimeout() to fire.
Added assert_properties
Merge pull request #88 from StephMc/codeReview

Added assert_properties
Merge pull request #93 from steveblock/testSeqSpeed

Update test-seq-speed.html
Merge AnimationListMixin into AnimationGroup

AnimationGroup is now the only user of AnimationListMixin.

No functional change.
Merge pull request #98 from steveblock/animListMixin

Merge AnimationListMixin into AnimationGroup