Check to make sure we can set another style.

Test for #3.
Add specs for setting of style.
Sass style should be a Symbol.

SafeYAML doesn't seem to like my symbols much. /cc @dtao
It seems to be read in as {"sass" => {"style" => ":compressed"}},
rather than {"sass" => {"style" => :compressed}}
Merge pull request #4 from jekyll/set-style