Two enhancements: directories and Ant task

[ricksbrown]

It would be great if we could target an input (and output) directory instead of a single file.

It would also be useful to have a native Ant task. This improves performance when building since the Ant java task incurs significant overhead.

To meet our immediate needs we have implemented both of these enhancements here but if there is interest from the vaadin sass-compiler project it would be preferable to incorporate this functionality directly into the core product. Let us know and we could work on contributing these features.