Consistent file handling

Currently the file handling is different depending of the `sourceComments` option.

- when sourceComments is `map` or `normal` it use `file.path` as parameter
- on all other cases it set `opts.data` to be the stringified file.

This make the `.sass` indented files to not compile with the default parameters. node-sass or libsass need to know the extension to properly compile sass file which is not the case with the stringified content.

With the first commit only there was 2 tests failing. I think it was caused by the fact that these 2 tests where the only one with no real files (the error was that the file was not found).

Let me know if I can make anything to make this pull request happen.