implementing strong_parameters support

Discussed quite a bit here:
https://github.com/gregbell/active_admin/issues/1731

The idea is that we have a strong_parameters branch of ActiveAdmin that would not get merged into ActiveAdmin master unless chosen as a basis for work on Rails 4 compatibility. However, others could depend on this branch in their Gemfile (info in the README).

I had another branch based on the original patch from the google list, but this request was based on suggestions I got from a ticket in the inherited resources issue tracker as well as suggestions from those on the thread above.

Unfortunately, I don't have enough time to spend on writing tests for it, but I tried to alter the tests to indicate that it is only compatible with Rails 3.2.

I know this is not the normal method of contribution, and I apologize. The original attempt was more friendly to earlier versions of Rails, but it seems like it makes sense to start to break ties with Rails pre-3.2 considering these changes. In addition, there may still be references to mass assignment security (attr_accessible) and mass assignment security whitelisting should be turned off, etc. in the Rails used for testing in this branch.

To reiterate: this pull request *should not be merged into master* unless more work is done to make it compatible with past Rails versions *or* could be merged into a rails 4 compatibility branch, but the branch itself may benefit others to be part of the main AA project as its own branch.

Thanks in advance for feedback.