Adding softError property to Job entity

It will be used to control whether a job failure should or should not short circuit it's dependency graph.
Extracts processDependencies and applies it to failed jobs if softError is enabled
Test coverage for softError feature

Testing that the field gets persisted and that it works as expected in the integration tests.
Indentation fixes
Fix: task isn't removed from zk when failure hanppens

Persisting the old taskId but not the new one when handling failed task
causes the problem.
Merge pull request #309 from depay/master

Fix: task isn't removed from zk when failure hanppens
Merge pull request #307 from uken/soft-errors

Soft errors
Adding support for Slack notifications
Merge pull request #310 from motusllc/slack-notifications

Adding support for Slack notifications
Adding Mesos slave hostname to environment
Merge pull request #311 from motusllc/hostname_envvar

Adding Mesos slave hostname to environment