Inherit project permissions from more than just All Projects

Add the basic functionality to build a tree of projects for easier
administation of access rights.  With this change, any project can
act as a parent project to another.

The way to set a project as a parent of another is done with a
new command: `gerrit set-project-parent`.  Right now there is no
possibility to set or remove the parenthood from the UI.

Bug: issue 273
Change-Id: Iac514de89e24b470339ea53065f8b470de68ab75
Uploaded-by: Ulrik Sjölin <ulrik.sjolin@sonyericsson.com>
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/cache/SelfPopulatingCache.java

@@ -121,6 +121,11 @@ public abstract class SelfPopulatingCache<K, V> implements Cache<K, V> {
     }
   }
 
+  /** Remove all cached items, forcing them to be created again on demand. */
+  public void removeAll(){
+    self.removeAll();
+  }
+
   public void put(K key, V value) {
     self.put(new Element(key, value));
   }