Fixes #196
Return InvalidAppMaster if Class.forName fails. This message will be stored in the AppManager.appMasterRegistry and returned to the client when the client issues a ShutdownApplication(appId) so the client knows why the appmaster failed. Note we need to account for some sort of history server so an audit log exists similar to YARN's history manager and tracks all appmasters execution. This will be addressed is a subsequent issue opened.
Merge branch 'issue_196_1' of https://github.com/kkasravi/gearpump into issue_196_1

Conflicts:
	core/src/main/scala/org/apache/gearpump/cluster/AppManager.scala