Release 3.0.0 (#58)

* Ability to abort running task/workflow (#52)

Features:
* API to terminate a task/ job
* Do not enforce max execution time on a task
Changes:
* Ability to publish control messages from the scheduler to all the executors
* Task handler contract changes to allow stopping a task
* Ability to abort a running task/ workflow
* Tests - Remove timer based approach to check the job and trigger completion

* Release Kronos 3.0.0-RC1

* Ability to retry a task on timeout (#53)

* Ability to retry a task on timeout
* Fix issues with control message on retry
* Integration test to test basic restart scenarios

* Updating version to release 3.0.0

* Update java version in travis.ci to openjdk8

* Update version in Dockerfile to 3.0.0

Dockerfile

@@ -1,6 +1,6 @@
 FROM openjdk:8u151-jre-alpine
 
-ENV KRONOS_VERSION 3.0.0-RC2
+ENV KRONOS_VERSION 3.0.0
 ENV KRONOS_HOME /home/kronos-${KRONOS_VERSION}
 ENV MODE all