Move CoroutineName to common module.

Fixes #570

Author: qwwdfsad

core/kotlinx-coroutines-core/src/CoroutineName.kt renamed to common/kotlinx-coroutines-core-common/src/CoroutineName.kt

@@ -4,8 +4,7 @@
 
 package kotlinx.coroutines.experimental
 
-import kotlin.coroutines.experimental.AbstractCoroutineContextElement
-import kotlin.coroutines.experimental.CoroutineContext
+import kotlin.coroutines.experimental.*
 
 /**
  * User-specified name of coroutine. This name is used in debugging mode.