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/ExecutorsTest.kt b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/ExecutorsTest.kt
index 71b3f35..0ae885d 100644
--- a/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/ExecutorsTest.kt
+++ b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/ExecutorsTest.kt
@@ -31,7 +31,7 @@
         runBlocking(context) {
             checkThreadName("TestThread")
         }
-        context[Job]!!.cancel()
+        context.close()
     }
 
     @Test
@@ -40,7 +40,7 @@
         runBlocking(context) {
             checkThreadName("TestPool")
         }
-        context[Job]!!.cancel()
+        context.close()
     }
 
     @Test
@@ -63,7 +63,7 @@
             }
             checkThreadName("Ctx1")
         }
-        ctx1[Job]!!.cancel()
-        ctx2[Job]!!.cancel()
+        ctx1.close()
+        ctx2.close()
     }
 }
\ No newline at end of file
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
diff --git a/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/ConflatedChannelCloseStressTest.kt b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/ConflatedChannelCloseStressTest.kt
index 0025df7..fbc55bc 100644
--- a/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/ConflatedChannelCloseStressTest.kt
+++ b/core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/ConflatedChannelCloseStressTest.kt
@@ -35,7 +35,9 @@
     val pool = newFixedThreadPoolContext(nSenders + 2, "TestStressClose")
 
     @After
-    fun tearDown() { pool[Job]!!.cancel() }
+    fun tearDown() {
+        pool.close()
+    }
 
     @Test
     fun testStressClose() = runBlocking<Unit> {