Allow reupload of same file once the upload is complete

[karan-kang]

Adding an option to override the default behavior of ignore reupload of the same file.

[AidasK]

Tests ar not passing because you are using opts instead of this.opts, please fix and it will be merged. Thanks

[AidasK]

Could you add documentation for this option to the readme?

[karan-kang]

Added documentation for allowDuplicateUploads option.
#113

[lucretiu]

Could you please add this merge into the version in the dist directory? Installing Flow using bower ignores the src directory, which actually contains the Flow version that allows this behaviour. I spent an hour or so trying to find why duplicate downloads were not accepted, only to find out that the version I have installed, even though it's "the latest" one, is not the latest one :(
It's even more confusing because the README which is shown on github says that the option is available.

[AidasK]

Sorry for this confusion, I will try to release this as soon as possible.

[soccerloway]

Hi, I need using this option in my project, but I found this option didn't merge into the version in the dist directory. So I tried to use the version in the src directory. Bad luck, it throw an error said ' domNode.appendChild is not a function '. I also use ng-flow.js in my project.
Help me, thx~!