SSL-capable ServiceEmitter.
Replace google-http-client imports with real guava imports.
Fix confusing error message in HadoopIndexTask
Merge pull request #928 from metamx/ssl-emitter

SSL-capable ServiceEmitter.
Merge pull request #930 from metamx/errorMsgFixHadoopIndexTask

Fix confusing error message in HadoopIndexTask
Merge pull request #929 from metamx/google-cleanup

Replace google-http-client imports with real guava imports.
Use thread priorities. (aka set `nice` values for background-like tasks)

 * Defaults the thread priority to java.util.Thread.NORM_PRIORITY in io.druid.indexing.common.task.AbstractTask
 * Each exec service has its own Task Factory which is assigned a priority for spawned task. Therefore each priority class has a unique exec service
 * Added Task.Priority enum
   * Can be either NORMAL or LOW
 * Add options to ForkingTaskRunner
    * Add "-XX:+UseThreadPriorities" default option
    * Add "-XX:ThreadPriorityPolicy=42" default option
 * AbstractTask - Removed unneded @JsonIgnore on priority
 * Added priority to RealtimePlumber executors. All sub-executors (non query runners) get Thread.MIN_PRIORITY