Revert "Use exclusive lock in :ruby filter"

This reverts commit 68977f3368941a5569c9345d3e9012f8184b0419.

Thread.exclusive isn't available in 1.8.7, and doesn't prevent other
threads running in 1.9 anyway.
Don't redirect $stdout in :ruby filter.

Redirecting $stdout isn't thread safe.

Provide `haml_io`, an IO object that writes to the buffer.