Bug 835876 - Add the ability to restart downloads. r=enn

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extra : rebase_source : 0b7c0a7120b794410d01ef4e77b74e264a5f467c
Bug 835880 - Implement the basic DownloadList object. r=enn

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extra : rebase_source : 15c2ba2132c87fdd633dc069a1e2ebef727c6392
Bug 835803 - Add tests for downloads whose size is zero bytes. r=enn

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extra : rebase_source : ed691306ee8976dabcb77a3947eec660a7a6e02e