Fixing opening from the HUD.
Fixing typo'ed function name.

Fixes #343.
Fix for changes to chrome.debugger.
The latest Chrome prevents multiple debugger event handlers.
This is a good change, as in the case of many tabs being traced this
reduces overhead somewhat.
Added context pool and tests.
Make getContext return null if context type not supported.
Fixing some bugs in cancelAnimationFrame naming.
It was also possible to cancel random timers on browsers that didn't
support cancelAnimationFrame.