firmware: rffc frac, if gain
lib: if freq
fw: ack
firmware: if precision
firmware: automagically update freq when changing the if freq
fw: dc offset correction fix

pins b7 & b6 are floating, disable re-triggering
selecting a different HPC_STOP fixes the dc offset drift
disabling and enabling rx/tx after tuning triggers the dc offset
correction
firmware, lib: update comments, doc
Merge pull request #79 from Hoernchen/master

This is excellent, especially the DC offset fix.