Add support for sbt-assembly to zipkin-{web,{query,collector}-service}

Since the assembly plugin is already included, I assume this is not very far from the intended purpose. The code is longer than strictly necessary and has some repetition to make sure the change has only minimal impact. This can of course be changed by just adding everything to `defaultSettings`.

The conflicts for which I added the `first` merge strategy:
 - `com/twitter/zipkin/query/adjusters/*.class` built for `zipkin-query` and `zipkin-query-core` differed
 - `com/twitter/common/args/apt/cmdline.arg.info.txt.1` differs between `com.twitter.common.zookeeper/server-set/jars/server-set-1.0.72.jar` and `com.twitter.common/args/jars/args-0.2.6.jar`
 - `commons-beanutils-core` has both versions `1.7.0` and `1.8.0` somewhere in the dependency tree; I didn't track down where exactly.