Add ability to change advertising rate

[johnnyman727]

It would be nice to be able to set the advertising rate of a BLE module acting as a beacon.

To do that, we need to add a function to messenger.js which outlines the raw packet that is being sent over UART to the module. It will use the bglib.gapSetAdvParameters function. The function expects a 16 bit number describing the min interval, a 16 bit number specifying the max interval, and a bit mask to specify which advertisement channels are used. The interval argument can be between 0x20 and 0x400. The advertisement channel should probably be 0x07 to continue using all three channels.

Then, this messenger API function should be surfaced to the BluetoothController object in index.js.