Changes:

Added APIs:

Slb:
Complete slb.aflex.* class with Multiplart upload
Complete slb.class_list.* class with Multiplart upload, fix json with quotes in string_list values.
Complete slb.server.* and slb.server.port.*
Complete slb.service_group.*
Complete slb.virtual_service.* class

System:
Complete system.log.* class
Complete system.config_file.* class
Complete system.banner.*
Complete system.hostname.*

system.back
system.restore with Multipart upload
system.device_info.get
system.device_info.cpu.current_usage.get
system.show_tech.download

Nat:
Complete nat.* class

Network:
network.interface class
network.route.ipv4static.* class
network.interface.ipv4.* class
network.acl.* class

axapi_http.py:
* More "broken replies" added.
* Allow user defined header and payload to support Multipart
* Handle non json response
* raise on empty response and create artificial response

errors.py:
Add more errors mapping to support all the above.