Straightens out some sass_interface api issues

This has to be merged with https://github.com/sass/sassc/pull/72 to make travis ci happy again!

```
Moved `source_map_file` from context to options.
Fix for `omit_source_map_url` (propagate to cpp_ctx).
Removed superfluous `source_maps` from struct, generate
source maps when `source_map_file` string contains data.
```

With some of the latest patches the sass_interface API was changed. This created some issues for the perl bindings and the API is IMO currently not in a well defined state. My main concern is about the fallowing line that got added to sass_interface.cpp:

```
.source_comments(!c_ctx->options.source_comments)
```

We had some discission at https://github.com/sass/sassc/pull/69 and I decided to keep the option names for now, but straighten out the code logic. I guess the main issue was that source_map generating was linked to the source_comments option, which is IMO no longer needed as we can use source_map_file and omit_source_map_url for that.

But since `source_comments` was changed from int to bool, all bindings will need to update anyway. So IMO it would be a good if we would want to change the names to make their use more intuitive.

- To create source maps you need to set `source_map_file` (adds sourceMappingUrl to css)
- To omit the sourceMappingUrl in the final css you can set `omit_source_map_url = true`
- Enable comments about original line numbers by setting `source_comments = true`

This only lacks the possibility to add the sourceMappingUrl without actually generating the source map. But I guess this should never be a real problem.

Best regards
Marcel