Make svg attributes covariant.

And small related clean up.
Simplify functors and module signatures.

Introduce Make and MakeWrapped functors for signatures that will always
give the "good" signature out of a functor application. See Svg.M and
Html5.M for example usages.
Document signature functors.
Merge pull request #50 from ocsigen/simple_sig

Simpler functor signatures