documents the work-around for asset compilation error

Fixes issue \#15 and addresses the comments on https://github.com/jamesotron/faye-rails/pull/13#issuecomment-6688706 by documenting the mime-type config workaround.

This may not be the best solution for \#15 but it seems to work.

NB: I've also made the faye-browser-min.js.map updating part of the rake task to update faye js sources.