Support Swift 4.0 and 4.2

Turns out I haven't made proper releases out of tags, so I'm making one now.

Throw less errors

The previous version was throwing Swift errors left and right, which made it hard to use error breakpoints.

New version first validates advertisement packets and only tries parsing if the preliminary validation passes.

Fix RSSI=127 issue

When CoreBluetooth reports RSSI=127 it means it's N/A.

Report distance=inf when this is the case.

Fix TX power calculation for Eddystone

Apparently, Eddystone beacons report TX power value at 0m and not at 1m like other types.

This update fixes the issue.