add rake task for compiling .js.coffee.erb files into .js.coffee
fix erb compile rake task dependency
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
add publisher spec for custom channel buffer sizes