fix #75: detect `sass` binary in the path and use it if available

[drupol]

This PR fix #75 by doing:

• Use the binary sass if it is available in the path
• If the path is not available in the path, it will try to download it

[drupol]

OK will adjust my PR then ! Thank you for the review!

[bocharsky-bw]

Also, tests and code styles were fixed in #73 , please, release your PR

[drupol]

@bocharsky-bw Ready for another review.