Use Int16Array instead of Object array for LUTs and uses positive indexes.

This results in 4x speedup of the storedPixelDataToCanvasImageData and
probably 12x speedups for storedColorPixelDataToCanvasImageData.
GenerateLut should also be much faster.
Merge pull request #4 from jpambrun/int16InsteadOfObjectArray

Use Int16Array instead of Object array for LUTs and uses positive indexes.
Added support for trigger an event when an image is loaded
Updated the event handling example to show how this works
- Thanks to @jmhmd for this idea
Updated code contributors list
Made first letter of CornerstoneImageLoaded uppercase for consistency
Added some notes on the performance optimization related to signed data
Changed eventData parameter for CornerstoneImageLoaded so it is consistent with the other events
Updated readme with guidance on code submissions and maturity of the project
oops fixed markdown formatting of last submit