Add supported fields to Authenticate struct type

[gillespi314]

Adds DeviceName, Model, and ModelName to Authenticate struct, which are required fields for an AuthenticateRequest according to Apple's documentation

[gillespi314]

This PR was intended for a fork and was mistakenly opened here.

[jessepeterson]

For what it's worth: the table-prefix thing is a feature I've thought about adding. Having a configurable/dynamic option for that might be useful. Downgrading the syntax to MySQL 5.x isn't something we'll likely support (though — I might be open to adding a mysql5 backend. Maybe. Maybe not.

As far as the actual fields — we don't need to support them because none of the storage backends use them and the Raw authenticate message is passed to any integrations (notably the web hook). As well, in mysql backend the Raw authentication message is stored on the device record. So anybody can use those fields if they wish without having to add them. Though I might be open to adding them if a storage backend (or integration) really needed them (like we do for e.g. serial number).

[jessepeterson]

Added #64 :)