Fix #283, accel calibration getting stuck/out of sync

Added in functionality for a countdown timer to let users know how long
they have left to continue the calibration procedure. Once it times out,
APM will automatically move on to the next step, and "continue" will need
to be clicked continuously to cancel out. Ardupilot pull req #1003
addresses this. This fix also adds in new LinkInteface functionality,
enableTimeouts() and disableTimeouts() to enable/disable auto-resetting of
APM on communications silence.