Feature/bulk docs apis

Overrides __iter__ method of dict in database.py so that it fetches docs in chunks from couch.

@evansde77 @dmannarino @ksnavely @jcounts 
@evansde77 Yeah. The question mark was meant to say: "this means that we either fetch docs from couch or not." But it sounds like I wasn't sure what it did ... will rephrase.