fix(client/js/juqery-plugin.js): #1017 - better support for non-jQuery callback syntax w/ jQuery plug-in

We were already most of the way there, but some logic had
to be adjusted.  Previously, if you included traditional
(non-jQuery) callbacks in your code when using the jQuery
plug-in, returning false from these callback functions would
be ignored.  This would be a problem if you wanted to, for
example, stop a specific file from being submitted by
returning false in your onValidate callback function.  This
should be fixed in this commit.
chore(*): bye bye coffeescript dev testing file

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chore(test/uploader-demo-jquery.js): re-introduce placeholders

[skip ci]
docs(*): closes #1017 - update docs to reflect this "enhancement".

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