add couch:fold/{3, 4}

fold all document in the database

Example of usage:

1> {ok, Db} =couch:open_db("testdb").
{ok,{cdb,<<"testdb">>,
         [{user_ctx,{user_ctx,null,[<<"_admin">>],undefined}}]}}
2> couch:fold(Db, fun(Row, Acc) -> {ok, [Row | Acc]} end, []).
{ok,[[{id,<<"5f480e40beb20ce551d848a8b5000df7">>},
      {key,<<"5f480e40beb20ce551d848a8b5000df7">>},
      {value,{[{rev,<<"2-6cc0258e6120c8cdff8a3c75930ce542">>}]}}]]}
3> couch:fold(Db, fun(Row, Acc) -> {ok, [Row | Acc]} end, [],
[include_docs]).
{ok,[[{id,<<"5f480e40beb20ce551d848a8b5000df7">>},
      {key,<<"5f480e40beb20ce551d848a8b5000df7">>},
      {value,{[{rev,<<"2-6cc0258e6120c8cdff8a3c75930ce542">>}]}},
      {doc,{[{<<"_id">>,<<"5f480e40beb20ce551d848a8b5000df7">>},
             {<<"_rev">>,<<"2-6cc0258e6120c8cdff8a3c75930ce542">>},
             {<<"_attachments">>,
              {[{<<"frais_enki.ods">>,
                 {[{<<"content_type">>,<<"application/vnd.oasis.op"...>>},
                   {<<"revpos">>,2},
                   {<<"digest">>,<<"md5-6DZZHRxjEI8Q"...>>},
                   {<<"length">>,30272},
                   {<<"stub">>,true}]}}]}}]}}]]}