Reorganize function instances

I reorganized all function instances in a similar way than scalaz as discussed in #86 . I still want to:
*   Add implicit not found annotation with indication where to find the instances define in Monocle
*   Add a paragraph in the Readme explaining how to deal with imports
*   Add a top level import: e.g. import monocle._ (I got some compile errors when I tried to do that)

I am also wandering if I should merge package std and scalazi. I find annoying that I cannot name the package scalaz but I don't know how to name "the package with std and scalaz instances".

Please don't hesitate to share your opinion concerning this refactoring.