release 2.6.4

Merge pull request azkaban#340 from hluu/master

Adding velocity-tools EscapeTool to able to escape certain data element to prevent XSS azkaban#339

Merge pull request azkaban#297 from hluu/master

Azkaban throws NPE when accessing non-existent flow - HADOOP-6388

Update README build badge.

Release 2.6.0

Azkaban Release 2.5.0

Release Candidate 4

Release candidate 3

2.5.0 Release Candidate 2

Release Candidate 1