[Bluetooth] Add RSSI to each Peripheral

[bsideup]

Hello! I am playing with the Bluetooth support, and for my experiment I need to access RSSI (signal's strength). It is a common field in nearly every Bluetooth library so I hope it won't be hard to add it!

[bsideup]

And while we are at the topic of Bluetooth... the manufacturer_data seems to be missing for some devices, while "native" code gives me correct data. Notably, Apple devices have it as None. Sounds like a bug.

[bsideup]

Update: got it working with objc_util (a.k.a. raw API access) using CBCentralManager. The code looks ugly tho, so I would still hope to have a higher level Python API for this :)