Support for threadId on v1.7.8

[Nicolas Roger]

Hi,

I have made a fork and a new branch to support the threadId field for the version 1.7.8.

Unfortunately, there is no branch with this final version in the repo, so I cannot submit a PR.

Here is the PR in my fork, if you want to see the diffs:

https://github.com/nicolasroger17/node-apn/pull/1

Tell me how we can work to add this to the repo.