Fix interpretation of path specified as response.file

It is not clear to parse a path specified `response.file`.

* `foo/bar.json` is interpreted as `path.join(this.basepath, "foo/bar.json")` (relative path)
* `/foo/bar.json` is interpreted as `/foo/bar.json` (absolute path)

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#	modified:   lib/stubcell.js
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Fix response.file path according to https://twitter.com/yosuke_furukawa/status/487102875386920960
Merge pull request #26 from muddydixon/fix_response_file_path

Fix interpretation of path specified as response.file