[SONARPLUGINS-3111] Added PreAuthentication to Sonar LDAP

Hi

I added preauthentication support to the sonar ldap plugin as requested in [SONARPLUGINS-3111] [1].

It can be enabled by adding the following property to the sonar.properties config file:

    ldap.preauthentication=true

By default preauthentication is disabled.

The HTTP Header in which the username is stored can be defined by:

    ldap.preAuthHeaderName=REMOTE_USER

Note that REMOTE_USER is the default.

Please tell me if you think there things to be improved or documented.

Thank you for pulling it into the next version of sonar ldap.

[1]: http://jira.codehaus.org/browse/SONARPLUGINS-3111  "SONARPLUGINS-3111"