Support for private callbacks

[dwbutler]

I defined some callbacks on my model. The problem is that the methods look like the public API, and will cause confusion and bugs when users of the API call the callback methods directly instead of the workflow methods.

(For example, it would be easy to mistakenly call task.assign instead of task.assign!)

My first instinct was to make the callback methods private, but this didn't work. Digging deeper, I found out this is because workflow doesn't look for private callbacks. This patch helps workflow find private callbacks.

[geekq]

I like the idea of incapsulation. Will merge in your improvement! I just wish more expressive unit test: instead of another_transition would name it some_private_transition_method in the test.

[dwbutler]

I agree about the tests! They were a bit messy so I just hacked something in. I'll take some time to create a more descriptive test case for this.

[dwbutler]

I added a separate test, and also fixed ruby 1.8.x compatibility.

[geekq]

Merged and tested with Ruby 1.8.7 + Rails 2.2.3 and Ruby 1.9.3 + Rails 3.2.2.

Thanks for your patience! ;-)

[dwbutler]

Awesome, thanks!

[geekq]

Now changed to "support for non public transition callbacks", see also #58