New failsafe code to stop sending pulses on failure.

This makes failsafe much easier to configure on the flight controller.
The previous behavior of setting all the channels to 1000 was a bit
confusing because the pulses my flight control was receiving were
already less than 1000, so going into failsafe actually raised all
channel levels until I adjusted subtrim on throttle to keep it around
1010.

With this code, it simply stops pulsing in such a way that matches what
the frsky d4r does (see below).  This is a well-known and unambiguous
path.

D4R losing radio contact:  http://i.imgur.com/8Xd27VQ.png

This code losing radio contact: http://i.imgur.com/lxGL4CT.png

Use a fixed RSSI percentage range.

Based on a bit of sciencing around what I thought would be good range, I
came up with some values to constrain "0-100%" with for translation to
ppm values.

http://i.imgur.com/X42wxL1.png

Auto binding when uninitialized and RSSI over PPM

The onState is backwards.