added support for include_docs='false' param on client.py all()

This is in regards to issue #38. Its not as elegant as handling all of the parameters in a single iterator but I'm not sure how to do that with the response you get from couchdb when ommiting documents. Its not a big deal to use requests manually and just query /_all_docs, since py-couchdb has it as a dependancy anyways, but this way the library will cover _all_docs in its entirety.