Kerberos UserDirectoryProvider

Changes/features:

- for 2.2

 o change license to new "Sakai Foundation" language (slt)

 o fix: compare entered username (userID) to cache (from scott amerson)

- for 2.1.x

 o uses configurable, in-memory caching mechanism to alleviate load
   on KDCs caused by WebDAV usage (slt)

- for 2.1

 o compatible with new UserDirectoryProvider methods (core)

 o uses "master project descriptor file" for project.xml (core)

 o can use sakai.properties settings to reduce dependency on working
   systemwide Kerberos JAAS configuration (slt)

 o documentation improved and moved into separate directory (slt)