BLE interface testing

Mocking a BLE peripheral would be useful for CI testing of ble2lsl. Hardware testing will be important but is not possible on TravisCI servers.

Implement a GATT server, something like:
https://medium.com/jumperiot/testing-a-ble-gateway-ecbdabee6b67
https://github.com/Jumperr-labs/python-gatt-server