Make validator JARs hot-deployable, so when upgrading validators, there's no need to restart Azkaban webserver.
Address the review comment regarding a potential NPE.
Merge pull request #342 from Victsm/validator-plugin

Make validator JARs hot-deployable, so when upgrading validators, there'...
XmlValidatorManager should not throw an exception if the validator dir does not exist.
Update XmlValidatorManagerTest to reflect the modifications in XmlValidatorManager.
Merge pull request #345 from Victsm/validator-plugin

XmlValidatorManager should not throw an exception if the validator dir d...