Update SinkSubscriber behavior and thread usage

[BenWhitehead]

• Update SinkSubscriber to process callbacks on compute thread
• Update non 202 response handling of SinkSubscriber so that it follows the same pattern MesosClient uses when subscribe calls. Primarily regarding how the response body is handled. (If the response is 'text/plain' the body will be read as a UTF-8 string and set as the message property of the MesosClientErrorContext in the exception).

Fixes #58

[BenWhitehead]

Manually test thread fix by running:

> java -jar mesos-rxjava-example-framework-0.1.1-SNAPSHOT-jar-with-dependencies.jar http://localhost:5050/api/v1/scheduler 2 '*'
2017-02-16 21:07:01,616 INFO  [main                    ] c.m.mesos.rx.java.MesosClient        - Probing Mesos server at http://localhost:5050/redirect
2017-02-16 21:07:01,811 INFO  [main                    ] c.m.mesos.rx.java.MesosClient        - Using new Mesos URI: http://localhost:5050/api/v1/scheduler
2017-02-16 21:07:02,167 INFO  [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.Sleepy  - Launching 2 tasks
2017-02-16 21:07:02,243 DEBUG [RxComputationScheduler-5] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-1" }, value : TASK_STAGING)
2017-02-16 21:07:02,244 DEBUG [RxComputationScheduler-5] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-2" }, value : TASK_STAGING)
2017-02-16 21:07:02,348 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-1" }, value : TASK_RUNNING)
2017-02-16 21:07:02,350 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-2" }, value : TASK_RUNNING)
2017-02-16 21:07:17,596 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-2" }, value : TASK_FINISHED)
2017-02-16 21:07:17,597 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-1-1" }, value : TASK_FINISHED)
2017-02-16 21:07:17,846 INFO  [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.Sleepy  - Launching 2 tasks
2017-02-16 21:07:17,849 DEBUG [RxComputationScheduler-4] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-4-3" }, value : TASK_STAGING)
2017-02-16 21:07:17,849 DEBUG [RxComputationScheduler-4] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-4-4" }, value : TASK_STAGING)
2017-02-16 21:07:18,095 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-4-4" }, value : TASK_RUNNING)
2017-02-16 21:07:18,096 DEBUG [RxComputationScheduler-3] c.m.m.r.j.e.framework.sleepy.State   - put(key : { value: "task-4-3" }, value : TASK_RUNNING)