setting pillow to have a couch_db property settable at constructor

lets it be patchable in a more intuitive way as per @droberts suggestion.

addresses case: 68652
fixing overrideable init for BasicPillow

overrideable doc_class if need be, for tests, etc.
Merge pull request #39 from dimagi/couch_noattr

setting pillow to have a couch_db property settable at constructor