support for standalone mode (i.e. no Dexcom app)

[thebookins]

This PR

• Merges recent changes to master back to dev
• Updates Example app to swift 4 and fixes some CocoaPods bugs
• Changes auth procedure to use write then read, rather that write and notify. This is in line with xDrip+ and xdrip-js. Notifications do not appear to work for the auth characteristic, perhaps as a result of a firmware change.

Should address #65, #55 and #49.

To test

• run pod install from Example directory
• open Example/xDripG5.xcworkspace
• select a provisioning profile, connect iOS device and build

[ps2]

Thanks. Most of this has been addressed in #67. In the future, please submit any independent changes as separate PRs, and we can tackle them individually.

[thebookins]

Thanks. I'll try out dev with #67 and submit any further changes separately.