Add ability to change transmission power

[johnnyman727]

The feature to change the transmission power has been requested on the forums.

To do this, a function needs to be added to messenger.js (the agent that sends and receives packages from the BLE113) to send the correct packet. To do that, call the bglib. hwSetTxPower function and pass in a value from 0 to 15 to sent the transmission power. 15 is the highest. Then, expose the function in index.js, where the BluetoothController will ask the messenger API to send the message and receive the response.