Fix imported sass not updating

I've made a few additions to help with --watch mode. This PR updates the generator to keep track of sass files that also start with underscores, if any sass files are modified it triggers a full sass update. This corrects an issue where file `a.sass` imports file `_b.sass` updating file `_b.sass` does not cause `a.sass` to re-render. 

Other updates
* Fix - compile_in_place may not find the correct path if the destination is not in the `_site` directory
* Added - deploy_style key to config so that building can automatically generate a different style css file (ie. expanded for watch mode and compressed for deployment)
* Removed - syntax key from config. Syntax is now inferred from the file extension (just like the sass gem cli)
* Documentation updates

Anyway, I've found these changes to be helpful with my current jekyll setup. Let me know your thoughts.