path.Join replaced by filepath.Join

Here is a proposal to fix #4.
path.Join does not manage Windows separators correctly. From the official Go code, path.Join is:
    return Clean(strings.Join(elem[i:], "/"))
while filepath.Join is:
    return Clean(strings.Join(elem[i:], string(Separator)))
filepath.Join is the one giving the expected behavior in the case of the Revel tool.