No Job in newSingleThreadContext and newFixedThreadPoolContext anymore

* This resolves the common issue of using `run(ctx)` where ctx comes from
either `newSingleThreadContext` or `newFixedThreadPoolContext`
invocation. They both used to return a combination of dispatcher + job,
and this job was overriding the parent job, thus preventing propagation
of cancellation. Not anymore.
* ThreadPoolDispatcher class is now public and is the result type for
both functions. It has the `close` method to release the thread pool.

Fixes #149
Fixes #151
diff --git a/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/BroadcastChannelMultiReceiveStressTest.kt b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/BroadcastChannelMultiReceiveStressTest.kt
index 8b087ca..220afbe 100644
--- a/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/BroadcastChannelMultiReceiveStressTest.kt
+++ b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/BroadcastChannelMultiReceiveStressTest.kt
@@ -52,7 +52,7 @@
 
     @After
     fun tearDown() {
-        pool.cancel()
+        pool.close()
     }
 
     @Test