relative path support

[hqingyi]

relative path is supportted ，and pass tests on windows & osx。

    {
        "SublimeGrunt":
        {
            "gruntfile_paths":
            [
                "app/static/demo1"
            ]
        },
        "auto_formatting": true
    }

[aceslick911]

Please approve this pull request, this is an excellent addition to the plugin and will fix issue's I am currently having with my complex project across different developer's environments where absolute paths are not valid.

[acetrand]

This solved my issues as well.