Doc and spec improvements

I fixed the errors in the docs I mentioned in the other thread, added an explanation on "shallow commands", i.e. commands w/o an `execute` method. (I thought it would be better to consolidate all doc changes in a single PR) If you don't like "shallow commands" feel free to invent another name.
I also added an explanation on how to use underscore to create a command "class". It saves a little typing and conforms with other "class" declarations.
Also, I noticed there was an orphaned variable in geppetto.specs, my bad, it slipped in with the previous PR.