method for mango query

[pirr]

I suggest implementation for mango query (only CouchDB 2) http://docs.couchdb.org/en/2.1.0/api/database/find.html.

I used django's filter implementation.
data = db.filter(field__subfield1__...__mango_operator = value)

data = db.filter(Q(field1__subfield1__...__mango_operator = value)
                          | Q(field2__subfield1__...__mango_operator = value)
                          & Q(field3__subfield1__...__mango_operator = value))

Work:

1. Combination operators: http://docs.couchdb.org/en/2.1.0/api/database/find.html#combination-operators, except: $elemMatch, $allMatch.
2. Condition operators http://docs.couchdb.org/en/2.1.0/api/database/find.html#condition-operators,
   not tested types: Object, Array-$size, Miscellaneous - $mod.
3. Sort, limit, fields and other request json object but need test http://docs.couchdb.org/en/2.1.0/api/database/find.html#db-find

I will work on update. In first I would add Array operators - $elemMatch, $allMatch.

[pirr]

I added support for $elemMatch, maybe $allMatch (not tested).
aaf0395

feats: [{subfeat: 1, ...}, {subfeat: 2, ...}]
db.filter(feats_L_elemMatch__subfeat__eq = 1)

feats: ['FOO', 'BAR']
db.filter(feats_L_elemMatch__regex = '(?i)fo')

"_L_" - define "feats" field as list
"elemMatch__subfeat__eq" - selector on "subfeat" field

This can be bad implementation. I will be glad to suggestions.

[djc]

As you can see here, there are some test failures caused by this PR. Can you look at those and make sure they no longer fail?

[pirr]

I fixed doctest. But travis log error:

couchdb.http.ServerError: (400, ('bad_request', 'Referer header required.'))

I not get this error in local test log.