New mixin use-baseline in vertical rhythm module

Discussed with @ericam here : https://groups.google.com/forum/#!topic/compass-users/NecGNw31Rzc

The mixin takes a variable argument list of base font size, base line height and changes the context for the given block.

@debug-grid-background needed an additional argument, as it relies on the current value of $base-font-size which may be the wrong one. I didn't find any other mixin of function to patch.

I also made $base-leader and $base-half-leader function since they can mistakenly be used in the alternative baseline context without taking it into account. If this change is a problem, we should may be warn the user in the doc ?

Concerning the tests, I just added the new mixin in test/fixtures/stylesheets/compass/sass/vertical-rhythm, is that enough ?