Related to issue: https://github.com/kanaka/noVNC/issues/205
Split out the function to load a single script to Util.load_script.
In order to get sequential load, when on IE set the script defer flag.
It is currently working on webkit and firefox but just in case also
set the script.async flag to make sure that scripts execute in the
order they are added. Scripts should still load in parallel.
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