fix race condition which causes redis output to lose some messages in batch mode

Scenario:
  - @flush_thread starts writing to redis (@redis.rpush)
  - main thread adds a new message to @pending
  - @flush_thread deletes this message (@pending.delete)
add myself to contributors list
use Stud::Buffer for batch mode

pending the acceptance of https://github.com/jordansissel/ruby-stud/pull/2
and a new release of stud.
Merge pull request #354 from alexdean/redis_output_with_stud_buffer

Redis output with generalized batch/flush