Implement Realtime read of PPG/pleth waveform

[z3ugma]

Resolves issue #2 Adds BLE command CMD_RT_DATA = 27 # 0x1B and corresponding response in the format of

137 bytes, a struct of the form <2BxBx2B5x125s which is interpreted as:

• 1 byte SpO2
• 1 byte Heart Rate
• null byte
• 1 byte Battery
• null byte
• 1 byte Activity/Motion
• 6 bytes ignore/null
• 125 bytes of PPG/pleth samples @ 125Hz, so 1 second of samples