Add "hrm" screen

Based on "speed screen", with hear trate instead of instant speed
Receive heart rate
Heart rate: Initial value & unit
Merge branch '1.4' into 1.4_HRM
Refresh heart rate layer
v1.4.0-beta4
Merge pull request #5 from jay3/1.4_HRM

Bluetooth Smart (BLE) Heart Rate Monitor (HRM)
Display live screen only when necessary
Version 1.4.0
Merge remote-tracking branch 'pebble_pb/1.4' into 1.4