Attempt at introducing basic player cleanup.
Responded to Steve's review comments.
Fix minor bug.
More review comments.
Merge pull request #416 from shans/playerCleanup

Attempt at introducing basic player cleanup.
Use fragments for more specific links to the spec in README.md

This requires switching to the WD, as fragments don't work in the ED.
Merge pull request #421 from steveblock/specUrls

Use fragments for more specific links to the spec in README.md
Changed event handler properties to use lists of callbacks
Added add/removeEventListener and handled the list of callbacks
Minor bug fixes
Added test and fixed up existing test
Adding handlers already present should not be added again
Updated interaction between addEventListener and onevent
Improved testing for onevent and addEventListener behaviour