Handle submodules and subattributes (dotted.named.things)

Not heavily tested (rand py.test on 3.3 and 2.7) but if you see the note in commit 29c4d36 I was unable to pass references to the module.submodule and object.get_argparser function. Possibly I was using the api incorrectly?

I think it would be nice to make the code for handling :module: and :func: a bit more robust. Maybe it would make sense to also allow passing a direct reference to an argparser object (i.e. ":func: main.argparser") by some extra checks in the code? I had to add a function to my CLI class to make this part work.