Just assembled 2nd version!

It is by all means superior to the first one. Fixed all mistakes and implemented several improvements. Most notable are:

- signal amplification is now working properly. Useful signal resolution is 12 bits (out of 16 bits ADC - some space reserved to prevent overflows)
 - 50/60 Hz noise filtering now works on-board, so device streams noise-free data on smartphone (since BLE streams is done only at 122 Hz rate, filtering on the phone wouldn't be possible)
 - heartbeat detection algorithm now works at 976 Hz resolution, so precision of R peak timing is much better (~2 milliseconds, but exact value will be clear only after a set of experiments)
 - and a set of small improvements - electrodes connectors are more convenient, LED is placed in a more visible place, charger works both in on and off states, analog supply can be turned off by NRF chip for sleep mode and overall PCB design is more clean, better shielded.