Improvements to initialization, disconnection, and callback structures

Version 0.7.0 introduces several API-breaking changes and improvements:

• Add StartMode to better encapsulate CBCentralManager initialization options
• Add WarningOptions to better encapsulate connect warning options
• Fix handling of expected and unexpected disconnections
• Add disconnect handler to handle auto-reconnect
• Remove all cancellation callbacks; use existing failure blocks and look for BluejayError.cancelled instead if you need to differentiate between a failure caused by cancellation versus a failure caused by other errors
• Return Bluejay Peripheral instead of CBPeripheral for connect, disconnect, and RSSI observer callbacks; so that you don't have to import CoreBluetooth in many of your files anymore, and you can also get a "better/more suitable" peripheral object to work with
• Fix a scan crash caused by not clearing the scan request's global timer
• Remove set privileges on all public Bluejay instance variables; they are all supposed to be read only from the get go, but a few were settable and that was dangerous