Make .default_async_handler method public and return a callable object.
Fix some style issues.
Allow users to define a failover handler when async_handler fails. Fixes #135.
Split Rollbar.default_async_handler into two different classes.

The users can now configure the async_handler in this way:

Rollbar.configure do |config|
  config.async_handler = Rollbar::Delay::Thread
end

Also allowed for config.failover_handlers.
Add Resque async handler.
Rewrite Rollbar.async_failover.
Don't re-raise exceptions when async handler fails or all failover handlers fail too.

Log errors instead.
Fix logging messages and dump payload when all failover handlers fail.
Add Rollbar::Configuration#use_thread method.

This method will set Rollbar::Delay::Thread as the async handler. It just use a Thread in order to process the payload.
Add convenience method to set Resque as async handler.

Now users can set Resque in this way:
config.use_resque queue: 'errors' # queue is optional.
Add doc for config.failover_handlers.
Fix typo in README.md. Change 'queue' to :queue.
Merge pull request #135 from jondeandres/failover

Failover handlers when async_handler fails