update requestAnimationFrame detection
Merge pull request #2 from james2doyle/optimize-raf

update requestAnimationFrame detection. Also closes #1.
enable callback function
add example
Merge pull request #4 from james2doyle/example

add example
Merge pull request #3 from james2doyle/add-callback

enable callback function
Add UMD to support AMD and CommonJS includes
Merge pull request #7 from jeffcarp/umd

Add UMD to support AMD and CommonJS includes