Extend AS::Logger if it has it.

This avoid unexpected behaviours when running apps on rails4.

As Rails used to monkey patch Logger, and it doesnt do anymore on rails
4.
see https://github.com/rails/rails/blob/4-0-stable/activesupport/lib/active_support/logger.rb#L46
Add tests for rails 3 and 4
Only run AS test when AS is defined
Remove 1.9.2 test
Always extend Logger
Merge pull request #3 from arthurnn/buffer_logger_rails_4

Buffer logger rails 4