Implement Stringer interface for DeviceState (#49)

* Implement `Stringer` interface for `DeviceState`

If device adoption fails the error message isn't very readable at the moment.

```
Error: unexpected state '10', wanted target '2'.
```

* Use `stringer`

unifi/devicestate_string.go

@@ -0,0 +1,34 @@
+// Code generated by "stringer -trimprefix DeviceState -type DeviceState"; DO NOT EDIT.
+
+package unifi
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[DeviceStateUnknown-0]
+	_ = x[DeviceStateConnected-1]
+	_ = x[DeviceStatePending-2]
+	_ = x[DeviceStateFirmwareMismatch-3]
+	_ = x[DeviceStateUpgrading-4]
+	_ = x[DeviceStateProvisioning-5]
+	_ = x[DeviceStateHeartbeatMissed-6]
+	_ = x[DeviceStateAdopting-7]
+	_ = x[DeviceStateDeleting-8]
+	_ = x[DeviceStateInformError-9]
+	_ = x[DeviceStateAdoptFailed-10]
+	_ = x[DeviceStateIsolated-11]
+}
+
+const _DeviceState_name = "UnknownConnectedPendingFirmwareMismatchUpgradingProvisioningHeartbeatMissedAdoptingDeletingInformErrorAdoptFailedIsolated"
+
+var _DeviceState_index = [...]uint8{0, 7, 16, 23, 39, 48, 60, 75, 83, 91, 102, 113, 121}
+
+func (i DeviceState) String() string {
+	if i < 0 || i >= DeviceState(len(_DeviceState_index)-1) {
+		return "DeviceState(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _DeviceState_name[_DeviceState_index[i]:_DeviceState_index[i+1]]
+}