Appease golint

# Appease golint

Previously there were around a few dozen items it complained about; this eliminates all of those issues.

```bash
$ GOPATH/bin/golint . | wc -l
       0
$ go test
PASS
ok      apns    0.013s
```
### Interface Changes

1. `FEEDBACK_TIMEOUT_SECONDS` is now `FeedbackTimeoutSeconds`
2. `APPLE_PUSH_RESPONSES` is now `ApplePushResponses`
3. `PUSH_COMMAND_VALUE` is now `PushCommandValue`
4. `MAX_PAYLOAD_SIZE_BYTES` is now `MaxPayloadSizeBytes`
5. `IDENTIFIER_UBOUND` is now `IdentifierUbound`
6. `TIMEOUT_SECONDS` is now `TimeoutSeconds`

### TODOs

- [x] code review (@garethstokes, @draaglom perhaps?)
- [ ] merge
- [ ] :beers: