Implement raw css imports

Also refactors the lookup code for better readability!

This is an attempt to fix one of our longer standing issues -> https://github.com/sass/libsass/issues/318
I tried to read all related issues on ruby sass to make this compatible!
This PR allows to import a `file.css` with the following syntax:

```
@import "file";
```

This will try to resolve the partial as normal, but will also look for `.css` files.
Previously this would produces an error if no file could be found.

The old behaviour is still active:
```
@import "file.css";
```

Will result in:
```
@import url(file.css);
```

So this should IMO be pretty much backward compatible!
If this is not acceptable by default I suggest to make this configurable!

@hcatlin what do you think?