Removed === from graphics replay files.

Removed the triple comparison operator from graphics replay files since the operator makes comparisons brittle. The substep index of a playback (which denotes the current call within the current step) now returns -1 instead of null if no current substep exists.
Merge pull request #393 from chihuahua/getRidOfTripleEquals

Removed === from graphics replay files.
Got graphics replay to work in Firefox 24.0.

Created an extension manager to check if desired extensions are supported. If an extension is not supported, the manager tries to find a related one that is supported by altering prefixes.