Add .gitignore.
Rename As_features.elt to As_features.atom (internal name).
Review Features API.

* Rewrite documentation.
* Change argument order (name first)
* ~default becomes optional (defaults to true)
* Sort As_features' implementation logically
Add Assemblage.Features.{of_pkg,of_pkg_pp}.
Merge pull request #40 from dbuenzli/features-improvements

Features improvements