add better Active Directory support - case insensitive username

In this time, I am using wiki 1.2 together with LDAP(MS Active Directory) authentication. Users's sAMAccountName is not always lowercase. The situation is similar as mentioned Srini Pulugurtha in XWIKI-238.

Before wiki 1.2 was ready to go to our production environment, I had to "fix" username handling. It means, I had to convert name from login page to lowercase together with sAMAccountName.

So I think, it would be great to add some property into xwiki.cfg which will indicate that username is case insensitive.

For example:
xwiki.authentication.ldap.caseinsensitive_username=1

I think, this will be very usefull for all potencial wiki users, who want to use LDAP authentication via MS Active Directory.

Please consider this wish
I am ready to test this new functionality, if you are not using MS AD.