webserver: print PDNSExceptions

previously they'd get handled as (...)
s/AhuException/PDNSException/ in docs
jsonstat: accept ints as strings or ints
json webserver: check for presence of command
json webserver: return masters as an Array
json webserver: add new 'zone' command

Replaces get-zone. Intended to replace zone-rest in the future, also
create/delete of zones should also go through this.
recursor json: put command into command var

Just like in the Auth.
auth webserver: fix invalid JSONP on errors
json get-zone: properly return error if zone doesn't exist
recursor json: implement zone command
recursor json: reimplement domains list using rapidjson

Note that this changes types of servers and rdbit.
json: Remove old get-zone implementations

pdnscontrol users must upgrade to at least 4c0e059731.