* Add extension for supporting kerberos security
- This PR adds an extension for supporting druid authentication via
Kerberos.
- Working on the docs.
* Add docs
* review comments
* more review comments
* Block all paths by default
* more review comments - use proper Oid
* Allow extensions to override httpclient for integration tests
* Add kerberos lock to prevent multithreaded issues.
* review comment - remove enabled flag and fix router injection
* Add Cookie Handling and more detailed docs
* review comment - rename DruidKerberosConfig -> AuthKerberosConfig
* review comments
* fix travis failure on jdk7
Gian Merlino
Nishant Bangarwa