Add remote control / system control capabilities

[klada]

Right now there is no button navigation (cursor up/down/left/right/...) available through this library. This would however be very handy so the device could be controlled through IP entirely - no more IR remote would be required for navigating the setup menu.

Denon AVRs do support the following actions through the Telnet interface:

• Cursor Up: MNCUP
• Cursor Down: MNCDN
• Cursor Left: MNCLT
• Cursor Right: MNCRT
• Cursor Enter: MNENT
• Info button: MNINF
• Option button: MNOPT
• Back button: MNRTN
• Open setup menu: MNMEN ON
• Close setup menu: MNMEN OFF
• Get menu status: MNMEN? → Returns either MNMEN ON or MNMEN OFF.

HTTP control using these commands works as follows:

# Move cursor up
GET http://IP:8080/goform/formiPhoneAppDirect.xml?MNCUP
# Open setup menu
GET http://IP:8080/goform/formiPhoneAppDirect.xml?MNMEN%20ON

Note on the setup menu: It seems like the state can only be queried through Telnet (MNMEN? command). The phone app apparently maintains the setup menu's state (open or closed) internally and does not issue a MNMEN? query in order to toggle it.