rename normal mode to "solo" mode, add "dispatcher" and "worker" mode

first stab, not tested, waiting for bundle install to finish
bundle update
fix specs
hand on queue name
expose queue
have worker not enqueue jobs
document modes
source_id, not build_id
heroku docs
fix typo
simplify scaling comment so people do not rely on my guesses [ci skip]
requeue jobs distributed under a wrong dyno count
do not mimic signature in Hub.new
meter all the things
fix enqueue missing method call
Merge pull request #30 from travis-ci/rkh-scalable-omg

[WiP] Make scalable