Make PaperTrail a Rails engine

I've read #216 and you might think it is unnecessary, but it does provide some benefits.

Gems like kamianri assume that every AR model is autoloaded on Rails. In other words, they assume AR models are all under `app/models/...` directory. With this lazy loading mechanism, we can provide some useful things. For example, kaminari allows to change page method name(`.page`) to whatever the user want. However, since `PaperTrail::Version` is loaded when `paper_trail` is loaded by bundler, it can't take advantage the mechanism. As a result, kaminari fails to define the right method(see: https://github.com/amatsuda/kaminari/pull/262).

It sounds like it is just a kaminari/paper_trail-only issue, but it also could happen to any other gems. To solve that issue, we can make PaperTraila a Rails engine. Could you think about merging this request?