clean up uri-vs-cap terminology, emphasize cap instances instead of URI strings

* "cap" means a python instance which encapsulates a filecap/dircap (uri.py)
 * "uri" means a string with a "URI:" prefix
 * FileNode instances are created with (and retain) a cap instance, and
   generate uri strings on demand
 * .get_cap/get_readcap/get_verifycap/get_repaircap return cap instances
 * .get_uri/get_readonly_uri return uri strings

* add filenode.download_to_filename() for control.py, should find a better way
* use MutableFileNode.init_from_cap, not .init_from_uri
* directory URI instances: use get_filenode_cap, not get_filenode_uri
* update/cleanup bench_dirnode.py to match, add Makefile target to run it

src/allmydata/dirnode.py

@@ -271,12 +271,15 @@ class DirectoryNode:
     def get_readonly_uri(self):
         return self._uri.get_readonly().to_string()
 
+    def get_cap(self):
+        return self._uri
+    def get_readcap(self):
+        return self._uri.get_readonly()
     def get_verify_cap(self):
         return self._uri.get_verify_cap()
-
     def get_repair_cap(self):
         if self._node.is_readonly():
-            return None
+            return None # readonly (mutable) dirnodes are not yet repairable
         return self._uri
 
     def get_storage_index(self):