current sensor support added

currentscale is in 1/10th mV/A
currentoffset is the offset of the currentsensor in mV
This outputs the current and mAh drawn via MSP,
whereas the current is in 0.01A steps instead of mA steps used in multiwii.
This way the maximum current is limited to >600A insteaf of 65A.
For multiwii compatability multiwiicurrentoutput has to be set to 1.