Suggested api changes

I think it would simplify the code to expose the following instead of `compile`:

```
def compile_string(s, **kwargs):
    # ...

# and

def compile_file(filename, **kwargs):
    # ...
```

and drop `compile_directory` (libsass has [hard deprecated it](https://github.com/sass/libsass/blob/3.1.0/sass_interface.cpp#L272-L275))
