collector: builder style configs

* `CollectorServiceBuilder` now is the way to build a collector service
* Add `Adaptive`, `Adjustable`, `CollectorInterface` convenience
  functions for more intuitive configuration files.
* Update `collector-dev.scala`
* Add `collector-zk.scala` that uses Zookeeper for sample rate
  configuration
* Add `collector-zk-adaptive.scala`, same as above but with an adaptive
  sampler

Author: @franklinhu
Fixes #225
URL: https://github.com/twitter/zipkin/pull/225