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
Merge pull request #89 from steveblock/parentNode

Make TimedItem.parentGroup read only