replace evergreen with Karma
Add coffee-script to testing dependencies
remove travis:prepare task
add ruby specs back to Rakefile and clean up tasks
Add support for per-channel buffer sizes

When no buffer size has been set by a publisher, the default buffer size
(100) is used.
Also renamed MAX_MESSAGES => BUFFER_SIZE for clarity.

This implements #35.
use X-Firehose_buffer-Size header instead of query param