add current sensor support to frsky telemetry

this lets the naze emulate the FAS100 current sensor and also transmits
the internally calculated mAh drawn via the fuel level data.
Merge pull request #137 from luggi/frsky-telemetry

add current sensor support to frsky telemetry, remove spammy function call
i-term zeroing fixed (thx Lux)
Merge branch 'master' of https://github.com/multiwii/baseflight