Add hook for thread safety

[StevePotter]

Thanks for creating such a great gem @lazebny. It's been really helpful. We have 2 main web apps. One is Rails and the other uses Cuba and Sequel. We added paper_trail to the first one but had no answers for the second. Your gem has made it possible for us to use paper_trail across both.

We encountered a problem when implementing this gem. Before, SequelPaperTrail.whodunnit and SequelPaperTrail.info_for_paper_trail were global and shared among all threads. This is a problem in a web server that serves concurrent requests in separate threads:

1. Request 1 comes in on thread A for user Paul
2. Server sets SequelPaperTrail.whodunnit to 'Paul'
3. Request 2 comes in on thread B for user Amy
4. Server sets SequelPaperTrail.whodunnit to 'Amy'
5. Request 1 changes a model tracked in paper_trail
6. versions table gets a new record with whodunnit of 'Amy' and NOT 'Paul'.

Corrupted data! The regular paper_trail gem has a mechanism to bypass this. So I did a quick fix, where SequelPaperTrail.whodunnit and SequelPaperTrail.info_for_paper_trail can also be a lambda or proc.

This allowed us to define a lambda that uses the current request as a context for getting the value for the current user ID. No more corrupt data!

I hope you'll review and merge this PR and publish a new version. Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling e1a51c1 on Vydia:improve-thread-safety into fae8c31 on lazebny:master.

[lazebny]

Hey @StevePotter, thank you for PR. Changes look reasonable to me and i am glad that you found this gem useful.

Would you mind to do:

• squash commits in one
• add a basic test for it

[StevePotter]

@lazebny sure. Sorry for not testing. I was being a bit lazy. I'll add tests and squash.