Base Client

A base client that mostly takes the client code from Connichiwa. There are still issues, though:

- For some reason BTLE crashes completely when we use a target central in updateValue:forCharacteristic:onSubscribedCentrals … we use nil for now
- Unlike iOS, OS X seems to have an MTU of 20 bytes per BTLE message. Funnily, sending more data causes BT to crash system-wide which can only be restored with a reboot. Since I wanted to rewrite the data-sending in Connichiwa for a while anyway, I will do that now so it respects the MTU and then bring the new code here